Wizards and Clerics change their prepared lists all the time. Because of this, a hotbar is simply the wrong approach for a spell UI.
Neverwinter Nights 1 has one because it's a real time game and as such it needs single click casting. The NWN hotbar UI is much more powerful and fast compared to BG3 (you can switch pages fast with CTRL / SHIFT / ALT) and STILL you run out of slots at higher levels.
It's really poor UI design to force the player to constantly keep organizing their own hotbar instead of just organizing the spells (and other stuff) automatically, sorted by levels and upcast and metamagic options.
The weird container UI for upcasting spells is awful in BG3 too. And the chain movement system is the most awkward I've seen in a party based RPG. UI is definitely a weak link in BG3.
