Use Proper D&D 5e stats and XP rewards with XP split.
No additional enemies and no removing enemies. Same exact number of bad guys everywhere.
Lift level cap.
Increase party to 6.
The game would be perfectly balanced.
Player tested via tabletop, which the game is based on and designed to mimick. It works. Encounters are fun and rewarding.
No rebalance necessary. All they'd have to do is what I just mentioned above as an option for players.
Option 1: Play 1-4 character party. All stats and enemies as is right now with current XP rewards. Option 2: Play 5-6 character party. All stats and enemies are set to D&D 5e with D&D 5e XP rewards. Same exact number of enemies as 1-4, just different stats and special abilities, like resistance for imps and intellect devourers and so forth.