You're not yet ready to fight three stone giants, but you'll fight three cloud giants, which are rated higher. Okay?
And yeah, clearly nobody on this thread but you "understands the problem" since all you get every time you explain is shrugs and puzzled looks. To everyone else, "everyone dies, paladin last" looks like a bad outcome that the paladin should want to prevent. To you it's a win/win situation for the paladin. Clearly there is something else going on here that no one but you gets.
No, there are a few that understand the problem. Some support the idea of narrowing the AC gap. I think that will help solve the problem, so I don't have a monster that does the appropriate or slightly lower damage to the paladin and far above expected damage to the other PCs.
Bounded Accuracy is built with the idea of accelerating damage and hit points balancing out the lack of high AC. In mechanical terms that means a higher CR creature will hit more doing more damage and the characters will do the same. The problem I'm having is this mechanic isn't working well because I have high AC variation in the party with a couple of heavy armor PCs (Paladin and warlock) very hard to hit and a bunch of low AC targets (Bard, sorcerer/rogue, archer ranger) very easy to hit. The AC variation is roughly 4 base and up to 8 with spells (in the case of the ranger and sorcerer/rogue) to 6 to 10 in the case of the bard. The cleric is between with a 19 AC with a shield. So when I go after the ranger or sorcerer/rogue, they die real quick because they get hit multiple times. When I go after the paladin or warlock/fighter, they don't get hit much and take very little damage. They can practically kill targets that easily defeat the rogue/sorcerer and ranger alone.
I have to make this fun for all of the PCs. Tactics for killing them isn't much help. I know how to do that and can do it any time like any DM. It's designing encounters that make fights fun for the group that is the challenge. Fights where if I go after the rogue and archer first, they don't die while the paladin and warlock/fighter bat easy cleanup because they barely get hit.
What I'm going to currently attempt is throw in some AC magic items to close the AC gap. 4 to 6 points of base AC in a game with this Bounded Accuracy mechanic might be a little high. I think keeping tighter AC ranges will help making encounter design easier for me.
Grappling is another situation that is pretty hard on casters and non-martials. If you don't have Athletics or Acrobatics, grappling is a near auto-success. If you have one of the skills, it's very competitive if not in favor of the PC with the skill. Casters do have a few means to escape like
misty step since grappling does not disrupt casting. I'm glad it doesn't or casters would be in real trouble.
I'll see how closing the AC gap works and go from there. If Grappling becomes equally problematic, I may have to do something about that. I'll keep adapting as I run into problems. I know you never played 3E/
Pathfinder, but this didn't happen in that version of D&D because casters had long-lasting, powerful defensive spells that equaled heavy armor and access to magic items that also gave them near equivalent armor class unless you were heavily built for AC. That tighter AC range doesn't exist in 5E. Caster defensive spells are short duration, limited by number of slots, and not as powerful. I might at some point in the future come up with some defensive features for casters other than spells. First I'm going to try to tighten AC variation with magic.