Nah, you're thinking WAY WAY WAY too much inside the box. We've been playing these sorts of games here for 25 years, at least. If you stick to really seriously sound military tactics 99% of encounters are trivial and the other 1% are simply unwinnable scenarios where you're party is so overmatched that no tactic would suffice.
This is hard to fathom. I've also been playing these games for 30'ish years and that's not at all what my experience or the data suggests. If you assume the other side is sticking to sound military tactics too then they must feel that the encounters are 1% trivial and 99% unwinable by your math. Give me a build and I'll present you with 3 common encounter scenarios that seriously test that build. If you're experiencing 99% cake walks and 1% unfair why would you bother playing this game? I would say if this is your experience you're playing with awful DM's.
The ghoul encounter example for instance. Any party that EVER closed with the ghouls would deserve to lose that encounter. Pure and simple. Go back and read your Art of War or your Clausewitz. First of all you never take on an enemy without completely understanding what that enemy is and what the terrain is. It is simply not the way to win.
I have read the Art of War and Von Clausewitz. None of that applies to DnD. If you always have the terrain advantage and always "know your enemy" you're playing a different game than me. And 95% of the rest of the DnD word. Turns out in more than half of the encounters in DnD the pc's are moving into an area already controlled and defended by the bad guys. The bad guys might have terrain advantages and traps on their side, pretty much guaranteeing the pc's are at a disadvantage not an advantage.
We always try to ascertain what we're facing to the best of our ability but sometimes in DnD you're placed in a situation without any warning and no method of retreat. In your 25 years of playing have your pc's ever entered an alley or cave for any reason? Been on a boat? Fallen in a pit? No party ever tries to close with the ghouls but the ghouls do try to close with you. Is this difficult to understand? You're in a crypt (wait let me ask have your pc's ever entered a crypt?) a large room 80' wide and 150' long with an altar at the end of the room... columns flank the walls at 10' intervals. As your party gets close to the altar suddenly the sound of shifting stones can be heard as 4 secret doors open and ghouls rush into the room from both sides. Sun Tzu never had to deal with ghouls or surprise rounds...
Once again if this encounter seems "outside the bounds" of normal DnD to you, we're probably not really discussing the same game and I would say you're playing the narrative story version.
In terms of the ghoul encounter, I don't know what the terrain was for that encounter, but lets suppose it is the typical sort of dungeon crawl situation where you open the door and the bad guys are right there in your face. You've already failed because you should have found out what was on the other side before you committed yourselves to the battle.
Ghouls can be silently waiting in the shadows. Ghouls can be behind things. Ghouls can be in the bottom of the pit. Ghouls can board your ship at night from the ghost ship. The assertion that you can always control the encounter in DnD is ridiculous and speaks to me of a game that's no fun at all to play. If your dm just rolls over and lets you dominate every encounter why would you bother?
There would be a variety of ways to do that. One would be to simply have a character with sufficient mobility to immediately disengage open the door while the rest of the party remains at a significant distance.
This is laughable. Many monsters out pace pc's so planning on a "safe distance" and "disengaging" really means that sometimes your scout faces 1-2 rounds of attacks all by himself. Be sure you wave good bye before he goes down the alley alone. When your sneaky elven ranger pads down the hallway but is facing high perception ambush creatures he's effectively isolated himself which in DnD is bad. What happens to him when the large stone block falls behind him cutting him off from the party?
We always do that. Another useful tactic is to create some blocking terrain between the party and the door before opening it. Trip ropes, caltrops, portable traps, and portable forms of difficult terrain are all quite feasible, as are fires, etc. All easily arranged by a sufficiently prepared team.
If you "always" do that, I would suggest that the DM is very weak in your campaigns or the scout player is frequently rolling up a new character. If the sum total of your dnd experience is opening doors and finding the monsters directly in front of you I think perhaps it's you that needs to think outside the box. How much firewood are your pc's carrying for these fires?
It also helps a lot to have various other preparations in mind, like some way to generate a significant amount of concealment to cover any needed retreat. Smoke is pretty easy to do that with. That can also degrade or deny an artillery monster its effectiveness while your ranged attacks take out the front line of the monsters.
smoke works both ways. you can't always retreat in DnD. Everyone tries to prepare for combat including the monsters in a well run campaign. It appears to me that you are playing against "dumb" monsters blindly waiting like zombies for pc's to spring the trap on them. Your campaigns must be brutally slow as the pc's endlessly build a smoky fire in every room prior to opening any doors. Does the smoke ever attract the monsters in another room who wind up cutting off your retreat and putting you in a "double encounter"?
You should also NEVER fight on the terrain chosen by the enemy. Always force them to come to you and control the parameters of the battlefield in your favor. Got a problem with an orc lair? Smoke them out. Another good tactic is to simply use attrition against the enemy. Sooner or later some of those orcs have to come out to find something to eat or do whatever it is they do. Suppose they all come boiling out looking for a fight? Superior mobility (say being mounted) will allow you to engage selected parts of the enemy force at range and deny them the ability to force you to a battle on their terms.
In heroic fantasy there are an infinite number of reasons why you might feel compelled to go forth into unknown terrain. "the screams of a young girl being tortured waft down the dark corridor". You can't always control the battle field. In real life you can control it less than half the time. How come we're not able to smoke out insurgents? Sometimes you have to go house to house. I've had a lot of training in house to house and room to room combat. I understand the principles and tactics that can be employed, no matter what you "think" it reality the tactics are designed to level the the playing field as much as possible but the defenders still have some advantages. you can't always mitigate them. To suggest that the pc's control the encounter in even 50% of the situations is kind of montyhaul'ish.
Now, can a DM create encounter situations that are impossible? Of course. But I promise you that if you give me any standard style published type of adventure and myself and the people I normally play with, we won't even normally take more than trivial damage anywhere along the way

Sure as heck won't absolutely require healing in 80% of these situations.
The published encounters have lots of trivial n to n-2 encounters. I disagree strongly that you'll not take more than trivial damage in the n+2 or greater encounters. Once again give me a build and I'll point you to a bad situation.
About the best a DM can do is throw a really tough single solo fast flying monster at you, like a dragon. Those can be tough, but they are by far the exception and can still be dealt with if you know exactly what you're up against.
your DM appears to be awful. Every tactic you've recommended can be employed by the monsters. Your pc's can be trapped in a dead end or surrounded and if you try and block the doors the gnolls can roll barrels of burning oil into them filling your room with smoke and giving you endurance checks to avoid penalties.
A lot of it depends on how well you could spread damage in the group (a lot of groups overdefend or make it too easy for monsters to themselves focus fire) and what ways you have to mitigate damage.
This is sort of anecdotal and straw man. i.e. a lot of groups play badly and don't maneuver. How do you prevent gnoll huntmasters who ambush a party as it crosses a bridge from applying focus fire? If you believe Abdul, you already knew that the gnolls were there and you deployed smoke grenades to cover your crossing. Perhaps you traveled 100 miles further to go around the gorge and avoid the ambush and let the escaping bandits get away with <insert import item or person here>.
For example, Armor of Agathys provides a nice buffer of temporary hp and I did use that on one combat and our defender does get 8 temp hp per combat... that said, no one would have dropped even without those temp hp. But basically having another striker instead of a leader often gives enough damage output to actually kill things.
You're using anecdotal evidence from one moderate encounter (which you're still not defining clearly.) as the basis of a flawed argument. Describe the party and the encounter, other wise it sounds a lot like you made this up. I repeatedly say "gnolls", "ghouls", "orcs", "gricks", "grells" in my examples. I'm using examples that are similar to encounters I've actually fought in so I know how they played out.
The ghoul example is a little rude, because ghouls are frankly broken (even more so than a normal level 5 soldier, much like needlefang drake swarms), but if you can funnel the ghouls into a doorway or corridor, use the flaming sphere to clog the way, that at least buys you some time and makes it manageable.
Ghouls are scary monsters. All soldiers are kind of scary and all monsters with stun are kind of scary, but I've faced these encounters and won so I don't think that you can say "broken". I think all soldiers are undervalued exp wise but when I use the ghoul encounter example I'm not using an N+4 to account for this. I give you a real world example that a normal build party can and has defeated and you call it broken because it destroys your healing/build theory. Try and offer support for why you think ghouls are broken. you're making random statements and asking us to take them as fact.
To be honest the real strength of the cleric in your example isn't necessarily the healing, but ready radiant damage and turn undead.
Actually your mistaken, after the one round of turn undead which gives the party a huge chance to take control of the battle and release the pressure of constantly suffering from effects, the big bonus of clerics are bonus saves. If I could only have one power from the three in a ghoul battle I would choose bonus saves over turn undead. radiant damage is a nice bonus too but it's the saves that keep the other members of the party standing. Healing is also pretty critical. Bottom line though, without bonus saves ghouls (and many many other creatures with stunning abilities) will eat you for lunch nearly every time. Stunning creatures and powerful undead creatures are not rare in DnD. The massive advantage clerics get in these kinds of situations is what makes them superior to warlords.
Okie, the group with two leaders... 5th level group, the encounters were EL 7. So both +2 - first was an elite 7, normal 9, some minions that provided status effects, and some terrain-ish effects (hostages and a fire), and the second was a level 6 solo with a level 6 hazardous terrain effect.
solo's are really bad examples. what percentage of encounters are solo's? Are solo encounter usually threatening? A level 6 solo is an N+1 encounter for level 5 pc's. The only solo I can find for level 6 is a young blue dragon which is threatening to level 2 pc's but sort of a joke for level 5 pc's. The best he can muster is 3 attacks per round while he's facing 5 attacks from the party. His attacks are pitifully weak +9 ATT d4+5 x 2??? Level 3 Orcs are more threatening. His breath weapon is even weak, three targets, average damage 12 per hit... This is a challenging encounter in your campaign? We're not playing the same game. Switch your 1250 encounter for two gricks and a grell. The monsters can now gain flanking, they have more attacks per round and all the attacks have better modifiers and better damage (the grell gets a second free attack if it hits with it's +12 vs fort attack) they can deploy a stun condition EVERY round not once in the entire encounter, they get two attacks per round that give ongoing damage, they have more hitpoints than the dragon and we only have 4 pc's not 5.
No one was in any great danger of dropping, though some people were certainly injured and some bloodying happened. Bastion of Health was used in one of the fights, cause it does 3W damage, but I don't believe anyone fell to 9 hp or lower (ie, that the temp hp mattered that much).
Of course no one was in danger, the encounter is a joke from a danger perspective. You're not really even making a point here.