What they mostly need is fewer plotlines that wait until the final act to be resolved. An issue with BG3's large roster of party members and the fact that you can drag them all along with you is that almost all of them have questlines that can be progressed during a single playthrough and culminate in the final act. Add to that all the things relating to other NPCs and it winds up being a huge laundry list before you even start adding on things unique to that act.
Yup. And it's not even hard to find examples of games that handle this structure better - like ME1 and ME2 both do, for example.
Either they need to abandon that "boatload of companions" concept they've been having in their last couple of games in favour of focusing upon a smaller roster of party members during any one playthrough, or have at least half those characters' personal plot lines be resolved prior to reaching the final act.
That was the original plan, and they only backpedalled on it because of extreme negative feedback about even the idea of it, and possibly due to internal testing.
Specifically, from the launch of early access until most of the way through early access, Larian were saying "Look, Act 1 is when you'll get all your companions, and then the companions you take out of act 1 with you will be the only real full companions for the rest of the game". They kept saying this, and they kept getting told "That's a bloody stupid idea, nobody is going to like it".
When they pulled it was a bit after they'd done some non-open playtesting with NDA'd testers (still mostly volunteers rather than paid QA it seems), and it's unknown if it was connected to that or related to other decisions.
Particularly, what might actually have made them change their mind was that they stopped thinking they were going to introduce all the "real" companions (which used to include Minsc and Jaheira as in act 1 and romanceable) during act 1. That seems to have happened around when they restructured the act 2 and 3 plot significantly to de-emphasize "how much will you become an illithid/will you oppose or aid the Dream Visitor/Daisy" and make it more about significant moral decisions in the plot and to do with the characters. This is also when the the Dream Visitor/Daisy stopped being a sort of tempting figures and became more of a businesslike one and got renamed to the Dream Guardian (some content was cut in the process including pretty significant stuff, like the area was the whole reason for the game's "Down by the river" main theme!), as well as changing it so the Dream person now always
supports you whatever you choose, just with more whinging in some cases, and also you have to support them too, no matter how evil and insane you think they are, because you auto-lose the game in some kinda-contrived ways if you do any of the things that directly oppose them before the very very last bit of the game.
Hell it could be that all of the reasons are true and interconnected, but whatever the case, most of the way through the early access they said they'd changed their mind and you'd be able to take all the companions you'd recruited with you. This is also probably why camp-based stuff in Act 2/3 was buggy and weird for quite a while after the game came out, because they likely hadn't smoothed out all the issues caused by suddenly going from just one party of companions to potentially all the companions being in camp. It's also why one of the events act 3 tends to be a bit of anti-climax (
kidnapping a companion - usually now it's someone you don't care about).
Long-term I think it's always better to have a "boatload of companions" because there will always be some companions people are absolutely psychotic freaks about (in a good way), but who are probably quite deeply off-putting for other people (Astarion and Marazhai for example), and you don't want to only have likely boringly mass-appeal characters, or even worse, enough annoying or boring companions that someone can't even fill a party without including them (I've played games like that, it sucks). So with a 4-person party you probably need 8-10 companions. But yeah resolve some of the main stuff with those companions BEFORE the final act! Sure have small bits of content for all companions in the final act, but not major-ass stories!