First of all I disagree about complexity of BG3 gameplay. I am starved for more stuff. I think UI struggles to handle the amount of spells and skills you gather, yes, but that's because current hotbar is garbage and not fit for purpose, not because the game is getting too complex.
I am not sure how many players would be comfortable with paying full price for a game which hold off a big chunk of valuable content for the potential sequels. In general making a game, and planning what you can do in years time is a bad approach. Larian should focus on giving best experience in BG3 (which I think they do) rather then holding up to sell more down the line. If they want to add content for the sequel, there is lot of stuff people ask for, that won't be in BG3 - like various addons that people keep refering to. The fact that BG3 sold well doesn't automatically mean that:
1) audience is satisfied
2) they will return for the sequel
See the series named Pillars of Eternity. First game sold very well. Not many returned for the sequel. If WotC and Larian consider doing a sequel (or another RPG) they need to deliver as hard as they can with BG3.
Limiting levels not only means more repetition in combat from player side, but also smaller roster to go against. If BG3 was 10-20h long than sure - handful of levels could be enough, but it's not and it shouldn't be for that price. Already in a fraction of content we have level4 feels low. To have lengthy campaign (I think your estimation is a bit high, but let's say 50-120h) you need content to fill it with. That means levels and enemies. While BG1&2 is probably the only example I can think of keeping the protagonist and not resetting the level, it also makes BG1 a weaker game. Having fewer levels is fine in BG2 where we start at reasonable level of complexity - not so much in BG1 where characters start getting stuff at the end of the adventure. Resetting level of protagonist is always an awkward thing to do, and therefore I favour RPGs that switch protagonists between titles - something that Larian has been doing themselves.
As to BG4&5. It's not only a question if players would buy those, but if Larian wants to do those. What if they fulfill their vision for the game in BG3? Sure, Ubisoft produces same game over and over again, but while dislike some of Larian's creative decision, they don't strike as a business run by suits. They clearly challenge themselves with everygame, and I wouldn't be surprised if they wanted to change gears a bit after BG3. Perhaps after they reach D&D audience with BG3, they will want to go back to D:OS, where they have full creative freedom over the IP.