After months without touching the game anymore, I just finished my first playthough modding the save games for a party of six. Turns out that even with the terrible Larian controls and the hideous chain/unchain system, the UI problems with the inventory and the visual glitches during conversations (with the extra companions overlapping each other) it was by far the most enjoyable campaign out of of the ones I had so far.
More classes to play around with, more companions bantering with each other or giving me quests or other minor things to do, a better flow during combat (especially the big fights with a lot of units), with more chances to intervene to do something instead of just watching 20 enemies in a row, plus the fact that six men killed enemies faster.
Of course, the downside is that the game was a bit easier for it, but I can't really say it bothered much, if at all. And it's something that could easily be addressed with a custom difficulty balance, IF Larian actually cared about it.
This sounds very promising. Thanks for sharing.
Larian's indifference to this request is disheartening, however, because I would so very much prefer that this be a built-in option. There is no way I would ever play this game without using a mod to increase my party size. But at the same time, I hate having to use any mods in my games.