IMHO The debate about a bigger Party is important when thers a decision about having one implemented in future allready. That seems not the case sfaik.
Try to change the deabte about the iteration/ Variationproblems which will come into gameplay, when you want more and more characters which all need background storys, behavior etc etc etc. All those small details which hugley change the experience of the game. Every difference in a character needs another interaction and what not.
The more Characters you have to support with full backgroundstory and all it includes storywide until end of game is making the development of BG3 an endless story.
It takes a lot of writing time and then corrections etc. Its taking a lot, and i say alot of time to make it propper and not generical story stuff.
You want believable storys and interaction in this game between all characters, events, dialogs, encounters and so forth... Then switch to this debate how to achieve it.
Because all else you can have allready in other games not so focused on Story, Dialog, believable interaction etc. BG3 is more like a good Book in the way of trying to stay at a good compromise in storythreads by not having too many characters in the story.
More quality over quantity is key in BG3
I want to see the game beeing released at the and of 21 or mid 22 the least. Bigger party pushes this Date much more into the future or it stays but then with cut content or less defined Backgrounds, reactions everything which is allready in game for the 4 people party and works great, wont be working the same for a 6 - character party. When you want it done in the same developement time.