This game should have been done on a different engine. I could say that making that decision could be too radical for Larian as they could just make use of the resources they already have, but I think the current state of the game proves exactly the opposite. The engine is not adequate for what they said they were going to do. Their core changes to make something "more fun" broke the combat systems completely. So they added workarounds to deal with them, but then they need workarounds for the workarounds. It's such an idiotic and moronic approach that I honestly can't imagine that they even remotely thought this through.
I don't think the engine is the problem (unless you want RTwP instead of turn-based).
Part of the problem, as I see it, is that a lot of Larian's design seems to be based on "that's what people liked in D:OS/D:OS 2, so that's what they want in BG 3 as well". Which is fine for the crowd that wants a D:OS in a D&D setting, but it alienates anyone who is looking for a D&D experience. I'm playing currently with the D&D rebalance mod enabled to get the game at least somewhat closer to what I want and refrain from using the D:OS elements. Not a perfect solution and not one I should have to rely on, but I don't see any other way currently as long as they don't change the system to more closely resemble 5e.
