OK. This is going to create a stir, but it's been something I've been thinking for some time. So, whether you like it or not, this is the truth:
This game, from start to finish, was created with a 5-6 member party size in mind. In truth, all they'd have to do to fix imbalance issues is to use proper D&D stats and to allow 5-6 party members. That would literally be the easiest fix. Stop nerfing the enemies and make them proper stat-wise and then allow 5-6 party members and use XP split to award experience points. Make players create 4 custom characters in single player mode or 1 each in multiplayer. The first character created (the host) is the default party leader.
That said, the Custom Characters could each have a default template: Cleric, Rogue, Fighter, Wizard. Each with a default race. Cleric is Tav, an elf. This is the default Custom Character as party leader. Then there could be Vim the (just throwing names out) the halfling rogue, Dorian the human wizard and Bogan the dwarf fighter. Customize them if you want or just accept and venture forth. Your choice. Either way, you have a base party of 4 and then can add up to 2 origin characters like Lae'zel and Shadowheart.
Make this the standard, use proper D&D stats, make it so you can switch out all the custom characters; even Tav in case you just like all the origin characters or whatever. Then the game would be perfectly balanced with very little work because every encounter is already built based on a 5-6 character party. Nerfing wouldn't be necessary, XP split would cause characters to level up at a more appropriate speed, and the game would function smoothly. If you want to party with less than 5, that's up to you. The encounters might be more challenging, but then you could set the difficulty to easier to balance it out.
Anyway, that's what I think would be easiest for Larian and more appropriate for this game, whether you think so or not.