It just seems to me like the the best (and honestly only good) option, since you will allways have problems with hotbar in this game, there is simply too much things you can have there ... this way you could have there only few spells you use the most (i mean why even bohter having Air, or Mage armor on hotbar, since you use it litteraly only once per long rest), and the rest could be casted from spellbook (aka. spells interface).
But why not have "spellbook" properly implimented into game UI? The goal is to do what you want to do with as little fuss and as little clicks as possible. BG3 ain't the first nor only DnD game. It's been adapted successfuly to PC for 20 years. The closes comparison is Solasta as it uses 5th edition as well, but I refrained myself from refering to it as I didn't play it myself just yet (I do love those snappy dice rolls - quick and informative). Still their UI seems to do just that and at least to me seems to provide better organization and better information with no manual player management