Really though the reason I think 6 is best is because it would basically force the issue on addressing a lot of UI jank, which a party of only 4 allows to sort of gloss over right now. Things that are inefficient, and which could be much improved get sorta hidden and walled off that way as the jank is offloaded from Party management to Camp management. I'm not sure peeps are seeing what we give up in terms of party comp by narrowing it down to 4. It has knock on effects into the whole Kit out and divvying up the Monty Haul or setting up your hall of heroes for that run. I'd trade 6 active party members on the scene at all times for a dozen at camp any day of the week. The active party is where they should put the focus.
That sense of satisfaction for decking out your core players for a given run, that charm of reordering the portraits in the leadership rankings, choosing who goes where in a formation. I'm all for hugs and kisses, but give me 6 solid fist bumps and a hand on the shoulder and I can replay this thing forever and ever and it doesn't become boring.
Six Portraits! Six person party! Six core attributes in Dungeons & Dragons
Once you get to 6 many more things become possible. It will help them to fine tune the UI and the benefits to other aspects of the game will stack.
ps. also, and you'll have to forgive my now getting quasi-mystical about it, but the thread is 113 pages long so these are sorta the remaining arguments, but I'd suggest that the D6 is foundational to this game. To D&D I mean and to BG. Our trusty six-sided polyhedron, the elegance of the cube - 6 Faces.
Right now in BG3 party comp we are at our simplest entry level polyhedron, the basic pyramid. I want them to lvl it up from 4 to 6, because that's a huge power spike in party comps. To do it right give us a 6 person party to recall the D6 from among 20 potential companions to recall the D20. This is a useful analogy, I think? lol. Obviously you don't nix the D4, but you save that for the Tactician mode, toss it in then and scale things like that. Working sorta middle out instead to hit all the marks and work all the angles.
pps. oh and one last thought, but to me variety and replay in party comp and customization is the single most important thing in a BG game. It supersedes stuff like tactical challenge in any given battle, because it gives the player a reason to start another campaign. If you don't have that, then the fun battles don't really matter. You need a reason to run it again right? That comes from party comp and companion variety. Then instead of thinking about it like roles in combat, we think about it more like "who to take along this time" and "hmmm how kit 'em out?" like to be the best they can be, for each of the potential companions on offer. Like with all the loot and all the various options provided by the campaign. There are easily 20 would be companions in BG3, and if we got the pick 6 in dodgeball from that, the combos would be sufficiently various and distinct to keep it humming for a good long while. My hope would be that the Definitive Edition of the game allows us to recruit more companions into the fold and then expand the party to 6. Or at least for one of the game modes. Then I will feel like the EA feedback on this subject was definitively accounted for hehe.