It all seems to be about character management. 2 more characters makes the item management system even worse than it already is. It's a huge pain point now. Imagine 2 more on the team.

That said, you shouldn't base it on janky item management. Fix that and adding 2 more won't hurt the game.

Oh, and about balance. But balance can be adjusted easy enough. Use proper 5e stats for one, and if you need to, add more enemies. There you go. Now 6 party members works.

The pluses for 6 more than outweigh the minuses.