Quickcast popup menu like in nwn2. Shows only prepared spells and +1, +2 etc tabs for up casting or metamagic.
The current system in BG3 is a horrible mess where you can't find anything because of all the possible variations cluttering the interface.
Or a menu like PoE or Pathfinder Kingmaker.
At level 4, my cleric already has a super cluttered bar with spells and there is even a few on hotbar 2. The only none-spell thing on my hotbars is the basic healing potion and even then I tend to just open the inventory to use any consumables. Easier to find what you are looking for.
They already do this with Warlock's Hex, to decide what ability check you want to grant disadvantage on (kind of a useless trait, unless you target Dex and do it before combat starts so they have disadvantage on initiative). All 6 options appear in a pop-up box.
They also have this on the Throw action button. You click on it and everything you can throw in your inventory pops up.
This would be an easy enough implementation for them to do. The trick would be the checking for available spell slots and showing or hiding them based on that.