By default, the game already makes you ridiculously overpowered by Act 3.
Heck, if you're even slightly aware of how to build D&D characters, you become ridiculously overpowered in Act 1.
This is all without level 20.
As far as making things more challenging... There are various Tactician Plus, Tactician Enhanced and Nightmare Difficulty mods that provide customizable scaling to enemies (Health, damage, actions, AC, saves etc)
At the very least, you'll want some health increases (No, you don't have to worry about everything becoming health sponges. Even with 200% increased health things will still fall over in 1-2 turns)
Also, I'd recommend using Honour Mode ruleset (If you don't want to be limited to 1 save slot, use Custom Difficulty to select Honour Mode scaling) as it makes bosses somewhat interesting.
