It's an ambitious thing to allow higher levels because of the difficulty scaling. However, there are mods that expand the base level in the game, and mods that give you (some) of the spells you should have, and mods that add those spells to scrolls, and mods that rescale the difficulty. There's even mods that add custom classes, buff existing ones, or give gear to classes rather lacking it.
All in all, it's the sort of thing best installed before starting a new playthrough. If anything breaks or is done in the wrong order, you might need a complete reinstall of Bg3, destroying all save games.
Ahh, cool! That's great there are so many mods. Too bad they might be unreliable, though. In that case, I'll probably end up waiting to see if Larian updates the game more... even though chances seem very slim the game would get any substantive updates like these
Originally the game was to be capped at 10th level, the extra 2 added late in the day.
Wow! I can't imagine only having 10 levels! Hard cap at 12 is bad enough (imo)

By the way, in D&D spell levels usually go up to around half the level of the caster. So a level 12 character can cast up to a level 6 spell. So that's why your spell levels only go up to 6
Interesting! Good to know