The only reason I'd want 6 companions is if I also get to create my own party. Right now I'm fine with four because I'm not keen on dealing with pre-made companions. While I like games like Dragon Age and Mass Effect, I don't necessarily enjoy all the companion interactions. Overly written companions control the narrative too much.. If I get to create my own party like in IWD then that's more fun for me because it's my own soap opera that I'm creating. I get to create the characters personalities and stories as I play the game. If it's someone elses characters then my brain goes all grumbly whenever they do something out of character for the character I'm trying to play them as.
like IWD, you could also create your own custom party in the original bg games too by essentially hosting your own mp campaign and filling in the party slots with self-made characters. i actually really enjoyed this being a part of those games too (including the black pits side-stories) as you mentioned it allowed the player to create their own party of custom made characters and then be able to act out the adventure and party dynamics as they went through the story, so i really hope larian does include this as an option too as part of the full game on launch - the og bg games were more restrictive on character creation than 5e, so it was really fun theory crafting all good/neutral/evil parties or just cool character concepts that i hadnt yet experienced in game (which you could also plan a party around when you had another 5 custom pc slots so even if characters werent 'optimal' you could still mold a party to fit your playstyle or pick up an appropriate npc companion among the dozen plus that both the originals had).
I also agree with what you were saying about playing with companions whom control the narrative too much or 'steal the spotlight' from my own pc - which i think the origin companion characters do to a certain extent. at this stage in the games development i dont think reducing the origin characters role is realistic or a good use of resources, however i do wish they would add some more game mechanics that made me feel like my own pc was more critical to the overall games plot, as currently it feels like if my pc never survived the ship crash they wouldnt even get a footnote in the history books, bc all the origin characters theoretically would just carry on with the story as they still would need to address the tadpole situation whether our pc was present or not, which to me just doesnt feel like bg - just for an example, in the originals if your main pc died you had to reload, no tpk required. im not saying that larian should implement this function, and 5e death mechanics present different tools for a dnd game today - im just highlighting that as an example of how the game's plot/narrative/story all revolved around your pc and your pc's interactions (that you decided and selected) with the environment/companions