The points at which the game transition between acts seem a bit arbitrary (mainly for Act I to Act II), and I don’t see a narrative or mechanical reason to lock us out of previous maps and quests. As far as I remember, previous Baldur’s Gate games didn’t have this kind of points of no return. Why do you think they did it? Do you like it?
I feel like level is a bit less important than people think, at least for BG3.
Lower level simply means less HP and fewer spells to work with. But your damage for the most part doesn’t change (except for like lvl 5 as an example). In this video, this guy solos Act 1. It does require more planning and tactics. But if you can solo this stuff, you certainly can do it with fewer levels. Just those additional levels will make it easier.
https://www.youtube.com/watch?v=N12VfAo6WNk
And here’s a video of them trying to beat the game (full team) as level 1
https://www.youtube.com/watch?v=kT-stzIRvGk