Lessee, each has an AC of 18 and blindsight out to 60 feet as well as darkvision out to 120 feet. So, right off the bat, there's no way for the party to sneak up on them (virtually) and they absolutely should be attacking from outside the range of the PC's.
We are not sneaking up on anybody at all generally (although my PC is almost always invisibile)
and the monsters we are fighting are way more powerful than Ice Devils ...... they are more powerful than 5E or 2024 Ancient Dragons are in fact.
Attack from outside the parties Range?? How?
The maximum range thing an Ice Devil can use goes 60 feet. That does no damage and simply puts the party or most of the party on the other side of it. Our fighter can close 60 feet using a bonus action. He can close 100 feet using a bonus action and action surge and he can close 140 feet if he can manage to throw a weapon and score an 18 on the dice (and he gets a reroll on that).
That is the fighter. Our Wizard and our Bard can BOTH Dimension Door and they can both take someone with them. My character (Rogue/Warlock/Paladin/Bard) is not nearly as fast. She is the slowest in the group and can only go 60 feet without using an action, but she can misty step over difficult terrain to do it while also being Invisible.
Oh and that 60 foot wall of ice does very little damage. You use that and you either put the whole party on the other side (so the Devil's can't do anything) or you put it between the party and split them so you have a few party members on the other side deciding if they want to use any resources to break it down or just let the folks on the side with the devils destroy the devils.
Like I said I don't think you have played much high level 5E.
Dropping walls of ice to divide up the party and break up sight lines.
We are regularly blinded, but I have a familiar, so for the cost of a bonus action I can be "unblinded" at will. The cleric has Lessor Restoration, which is a bonus action to cast. The Fighter has tremorsense.
So can we be blinded? Sure, we are quite regularly. But it is also pretty easily overcome at this level.
Sure, the party is taking half damage from the cold,
Which is like not relevant at all. We get 25 temp hit points every short rest from inspiring leader and every other player except the fighter gets heroic inspiration from Musician every short rest. The fighter used to get it too, except now he gets it every single turn.
but, I should be easily able to make this combat last 3-4 rounds without "kiting" or any shenanigans.
Against 18th level characters?
My Rogue casts Banishment with a DC of 21 and being invisible cancels that advantage they would have on that save.
Tasha's Hideous Laughter is also DC 21, again with disadvantage on the save to cancel magic resistance and I have a good shot of putting both of them on their butt with a 2nd level slot.
That is the PC who is the weakest caster in the group!
Oh, and let's not forget that my Ice Devils can Gate, potentially. And, with 180 HP each, it's going to take 2 or 3 PC's ganging up on 1 to kill it in a single round.
Not if they are dead or incapacitated ... which they will be.
Again, unless the encounters are all starting 20 feet away, in bright light,
More like 100 feet, not 20 feet.
PLUS resting after each encounter,
We are not resting after each encounter. Who said that was the case? Also when you start talking about Short Rests, have you read the 2024 spells?
Prayer of Healing is a 2nd level spell and gives up to 5 PCs a short rest plus 2d8 hit points. When we use it, we usually only use it on 1 or 2 characters who need the rest so we can cast it again on the other characters who will need the rest later.
When the Fighter runs out of Action Surge the Cleric casts Prayer of Healing on him after the combat is over. When the Cleric runs out of Channel Divinity he casts Prayer of Healing on himself. When the Bard runs out of Inspiration the Cleric casts Prayer of Healing on him. Finally, when my PC runs out of Warlock slots the Cleric casts prayer of healing on her and when he does I get my slots back plus everyone else in the party gets 23 temp hit points and a Heroic Inspiration. If it is a convenient time for the Wizard to use Arcane Recovery we throw him in there as well. This greatly enhances our ability to recharge short rest stuff and it is a cheap 2nd level spell!
Oh I do have Rope Trick as well so we could safely rest after every encounter if pressed, but we don't.
We actually did a short rest as a party one time in the 5 battles we had in the last dungeon we just cleared (after battle 4, before the Boss battle).
PLUS using 2014 encounter budgets.
We are NOT using 2014 encounter budgets. As I noted earlier we are playing MCDM Where Evil Lives which uses monsters mostly from MCDM Flee Mortals which are substantially more powerful than their 2014 counterparts and generally more powerful than their 2024 counterparts.
And, they are also getting this result from every single module?
Well I have only played one campaign from start to [nearly] finished .... so yes it is from "every single module" as it is the only one I have played in 2024 at high level.
I am playing in another 2024 campaign, but we are only 10th level in that one. So I really can't comment on high level play in that game. That is also an XP game and although it is very easy now it is because we are 10th level playing a 7th level part of the campaign, mostly because we got more xp than expected early.
Similarities in that campaign are:
Low level spells are more powerful than before
Low and mid level combat takes longer than before (not more rounds, more time)
Bastions are OP
You think it's far more likely that the system, where no one else is getting these results, is the problem, but, it couldn't possibly be the DM?
Do you have results to report from other people?
To turn this around though - Do you think it is the system or the DM when this group of DM and players have extensive experience playing high level in 5E and have NOT had similar issues to this degree?