I haven't followed BG3's progression for ages it seems. Is Larian's stance still firm regarding party size or is there some leeway judging from their comments (if they've had any on the matter)?

My personal preference leans more towards the classic BG party size. I haven't tried BG3 in a long while, but my feeling at the time was that the 4-party iteration they had then didn't cut it for me. Maybe it was just the whole shebang that didn't live up to my glorified expectations, but still, I think they should allow for 5-6.