Hi, Mike. Big fan.
I've run one campaign past 20th level and one campaign up to 13th level, so I feel your pain. The game changes around 13th level with spells like plane shift and raise dead. I don't think they break encounters so much as they break campaigns. When the villain decapitates a character's little sister, the cleric just shrugs and brings her back to life with a single action spell, the wizard casts feeblemind, and the fighter action surges. Bye bye, villain. Cue sad trombone sound from the DM.
Action economy is key. One of my groups has 3 PCs. Higher level combats are still challenging for them. But for my 5 PC group...they are nigh invulnerable. And I know you play with about 6 characters.
The fix is simple: Monsters need to hit really, really hard. Elite-style monsters should be able to drop 1 character per round, and Solos should drop 2 or 3 characters every round. Anything less won't even get the players' attention.
I have a formula for monster modifications. Basically, as the players level from Tier 1 to Tier 2, and Tier 2 to Tier 3, etc monsters get tougher. Yes, a Tier 1 goblin is tougher than a Tier 3 goblin. Here's how it works...
First, I set a minimum threshold for attack bonus. Tier 1 = +3, Tier 2 = +6, Tier 3 = +9, Tier 4 = +12. I add an additional +3 to Elite and Solo monsters.
Second, I increase damage. For Elite monsters, I take their average damage and multiply it by Tier. For Solo monsters, I take maximum damage and multiply it by Tier. And that's how much they do on every hit, period.
Third, I don't mess with hit points or armor class -- except for Elites and Solos. I take the attack bonus and add that to their AC. For hit points, I multiply them by Tier using the same formula as damage.
Finally, I've developed some additional Legendary Actions that I can apply as needed...
- Legendary Attack: One attack per tier. So a CR17 monster gets at least 4 attacks/per round
- Legendary Reactions: One reaction per tier. A CR17 monster gets up to 4 reactions per turn.
- Legendary Movement: Creatures may take a move action without provoking opportunity attacks at any time during the turn. They may do this a number of times equal to their Tier. This ability refreshes on a long rest.
- Legendary Accuracy: +1 bonus per Tier to all attack rolls. A creature can choose to turn a miss into a hit a number of times equal to their Tier. This ability refreshes on a long rest.
- Legendary Damage: A creature does maximum damage on a hit. They may do this a number of times equal to their Tier. This ability refreshes on a long rest.
- Legendary Critical: Critical range expands by +1 per Tier. So a Tier 4 monster crits on 15-20. Criticals double standard damage (since we are not rolling for damage).
- Legendary Recovery: Creatures may end one condition at the start of their turn. They may do this a number of times equal to their Tier. This ability refreshes on a long rest.
- Legendary Resistance: One resistance per tier. A CR17 monster gets 4 uses of legendary resistance.
- Legendary Defense: A creature can choose to turn a hit into a miss a number of times equal to their Tier. This ability refreshes on a long rest.