I remember how BG-2 worked. Although I used the Dragonspear UI++ mod. There were icons/buttons for specific groups of things, like spells, special abilities, and so on. Click one and you populated the HotBar with that category. There were times when you had to scroll through multiple bars worth of entries.
What I'm suggesting is something like a condensed Spells panel. Maybe there's an option to show everything or show what you specify and have others under a separate button. But I am trying to avoid menus within menus.
I can't remember what Divinity:Original Sin 2 did, but I don't remember having an issue with it.
DOS2 literally just has one hotbar for everything, pretty much what we have now in BG3, aside from the standard actions panel. You can switch between multiple hotbars, but there really is no organisation. The difference is, there is no upcasting in DOS2 and fewer standard actions. You don’t even really need to keep the standard actions on your hotbar because you can do most things with a right click on enemies, objects etc. You can have items in your hotbar which can get a bit messy, but there are few that you really need in there – and there’s the option of putting backpacks into the hotbar which gives you a popup to select the items within.