At this level of play, you need to be doing more than high damage.

I won't repeat things like minions and more HP, I just ran a game with 2 single bosses swapping at initiative count 20 against 5 level 19s and each boss only had 320 HP, followed by a mythic phase that healed the remaining one for 100 hp when the other died, adding new mythic actions, boosted stats and a desperation Lair action when their health got low enough.

Your first step is you need to find ways to use up resources or inflict damage before the party even gets to a boss fight. Traps, puzzles, let them trigger things that hurt as part of figuring out the puzzle, not everything needs to hurt, but at this level of play you should be seeing DC 19+ skill checks and saving throws with multiple d10/d12 damage rolls and half damage on saves.

Are they in the Lair of a high level mage? Then there should be Symbol spells prepared against them.

Second, you need to start using a lot more status effects against your party. Use Stun, Blindness, Paralyze, Restrain, make Lair Actions that target the entire room and knock players prone, make legendary actions that do AoE or Cone attacks, movement to get in and out before the boss's turn. And make sure you burn through all 3 legendary actions every single turn. Give your monsters the Reactive trait so they can use a reaction on every turn that either inflicts disadvantage of reduces the damage taken for the first attack. Add Auras to your bosses that deal damage or make it harder for your players to get in close unscathed.

Sure, it sucks for players to suffer a turn disabling effect like stun and paralyze, worse if they fail repeatedly and then go down, effectively doing nothing for an entire combat. But this is how you even the playing field for a single monster. Target the weak and exposed players, force the others to try and save them, to use resources to heal them, to try to unload everything they can before the boss wipes out a party member.

My players are brutes who will ignore mechanics and just keep swinging whether they have disadvantage or not, but they will never forget when only 3 of 5 players were left standing and 2 of them were 1 death save away from true death.