*I* want to have fun in combat too, so for me having to self nerf in combat because rolling out the full range of abilities on this terribly under CR'ed Centaur or Dragon will instagib the party in combat detracts from my fun.
Then don't "self nerf" once you have picked out what creature to use. Aim at lower CRs than the party level if you are afraid of "under CR'ed" creatures (which is a thing I've only heard people say about Bugbears, Ogres, Hobgoblins, and Intellect Devourers in 5th edition - most complaints about CR is that a creature is supposed over-CR'ed, and even those are just misunderstanding what CR actually means)
I like to play intelligent monsters all out - a style of play that's much harder to support with the serve yourself at the buffet style of play.
I genuinely have no idea what you are trying to say. I run intelligent monsters as intelligently as makes sense, I run my campaigns fully player-driven with me filling in what makes sense for what they are doing and where they are going. I have no difficulty in doing those things at the same time.
Secondly, I like combats to be dramatic and I *hate* filler fights.
I don't have any "filler fights." I have no idea what you even mean with that phrase.
The best combats are ones where the players are intensely challenged, but pull through in the end.
I find that to not necessarily be the case. Many of the most memorable combats to have taken place among my group are not the ones in which the party pulled through in the end, but rather ones in which it was obvious there was significant risk of harm to the party and they caught a lucky break on their strategy working out so that they won while relatively unscathed, or ones in which the party realized they were in over their heads and got away without anything too bad happening.
Thirdly, I don't like actually killing players.
Assuming that you meant characters, rather than players (which hopefully no DM is in the practice of killing), I agree. I don't like killing characters either.
I run my game as I've suggest you run yours, I never fudge, and I've seen no 5th edition characters die - they are just that difficult to kill without intentionally going for the kill by continuing to attack downed characters and prevent other characters from healing/stabilizing them.
...particularly if they get punked in a stupid battle.
The solution there is easy: Have zero stupid battles.
You can see with the things I want from the rules, I want good support for making well balanced combats before I put the miniatures on the battlemat, or sketch out the theater of mind situation. This
A) Ensures that I can play the monsters hard
B) The fight will be tough and dramatic requiring the players to step up
C) It's unlikely that I will accidently make it too hard - there is a related thing here about swingy systems. One reason I like 5E more than 3.5E is that's it's not as swingy once you get past level 3.
It's entirely possible to want different things from the rules though, so more power to you, but that's why I want to be able to prep reasonably well.
The only want that you are expressing that differs from my wants is that you want something besides "I'll just not use any creatures equal or higher CR to the party level" to follow as a guideline, even though there isn't actually a need for more than that to reach your other stated wants.
I mean, this is what I used to do, but what do you do if you screw up and the players are going to get rolled by this combat in a very unfun way, such as getting blitzed by a T1 glitterdust?
Let's use an example:
The party (3 characters, 5th level at the time) had just teleported via device onto a small island in the middle of a subterranean lake. The water was dark and murky, and they were expectant that some creature might lurk within. They tossed in a rock to see what response it might provoke, and a Hydra (CR 8) lunged forth angrily.
Initiative was rolled, and actions begun. The party tried to fight, finding in the first round that their attacks weren't likely to defeat the hydra before it's attacks finished them off. So in round 2, the party corrected their mistake of trying to fight such an overpowering enemy by retreating.
To state my point clearly: It is not by
my screw up that the players wind up in a situation that might spell their characters' deaths - it is by
theirs. And since we don't have any unfun or "filler" or "stupid" battles, each and every one is a success even when the characters suffer defeat (because defeat is dramatic and interesting, just like victory).
Or the lich has scry'ed them and is now going to execute a timestop powered 'die' phase on them?
If the party is facing a lich, they either have the capabilities to deal with a lich at their disposal and are responsible for using them correctly, or they have gone out of their way to agitate this lich to the point that its plan to deal with them has changed from "ignore them, they are insignificant, fleeting beings," to "personally see to their immediate destruction," while not having the capabilities to deal with a lich - which is to say either this isn't a problem, or it is a problem the party has deliberately worked towards for a not insignificant amount of time so it must be what the players actually want.
When you ran 3.5 did every mage from level 3+ heavily rely on save or dies?
Not that it has any significance to the topic actually at hand currently, but no, they didn't. I managed to have a group for the 3.5 era that didn't really enjoy playing spellcasters that often, despite the rules-set of the era heavily favoring such characters.
I mean, I had these problems and just didn't play the monsters optimally (I don't think I ever used the glitterdust/stinking cloud/black tentacles line of spells on players in 3.5), but we still had tons of bad fights caused by badly designed encounters (I can remember EXACTLY the first time I realised that dragons were under CR'ed because it killed two players).
You find dragons under CR'ed in 5th edition, or are you referencing what happened with 3.5 as if it has any bearing at all on how 5th edition actually plays?
I haven't found any of 5th editions monsters to be under, or over, CR'ed at this point - but I suspect that is mostly because 5th edition CR means "a party of this level that is rested and equipped shouldn't have anyone die in the process of defeating this creature", which is significantly different from the 3.5 CR which means "a party of this level should lose X specific quantity of resources when faced with this creature." The former being very easy to actually match, while the latter was nearly always inaccurate in practice.