I would like to add 3 additional, and in mine opinion better, solutions, that i readed here around ...
1) Interactive spellslots ...
That way you will get on hotbar each spell only once ... and you will determine its spell level by choosing spell slots you want to use.
Meaning you click on spell level II., and all your spells you cast since that moment will be level II. (if they have level II. ofc) until you click on different spell level.
Personaly i think this system could be improved with enabling right-click on spells to change spell level for single cast ...
For example Mage Armor ... there is no reason to spend level II. spell slot, since level I. spell does exactly the same.
2) Expandable hotbar ...
Right now we have 2 rows and they are geting full allready. If we could expand hotbar to another row instead of searching in it ... our hotbar space could be instantly doubled, tripled, or quadrupled ...
Not as elegant solution, but it certainly works.
3) Enable casting from spellbook.
That way you in fact gets button that will contain all your spells, and it is allready possible to modify Keybind to your liking.
My favourite one, to be honest ... maybe it would be more usable, if there will be some possibility to open spellbook of only selected party member, simmilar to when you open character specific inventory.