By god, Lore Mastery is insane. Int initiative and some double proficiency. Virtually any spell is good to have prepared, no matter what you're facing. Changing damage types can be done as often as needed. And every control or save based spell essentially gets ~+2 DC by hitting a bad stat once a short rest. Almost sorcerer levels of flexibility on preparing and boosting without the known spells problem (ok, no quicken, but yours always hits where it hurts anyway and arcane recovery pretty much subs in for sorcery points if you want it to). Then to top it off, the spellcasting bit of Wish at lvl14, but better (lvl 9 spells are available, but only 1 turn casts and you need to pay for ingredients). Still, an extra cast of Wish, three levels early. Wow.
If they said "All these elemental changers, save type changers and boosters only apply to the Fireball spell, and you only get a free bag of cookies at lvl14" it'd still be worth a look. But the way it is, it's friggen amazing.
There is so much useful stuff you can multiclass this with, and even feat it up with, but it's an absolute powerhouse by itself anyway. I know what my next character is going to be......
((I'd almost nerf this to "Choose one spell you know per spellcasting level that you can then apply these effects to. You can't change which spell it is for that spell level, ever." Then at least there's some decisions to be made, and it's still really good. Even "Pick *one* spell you know, you may change which spell it is at the end of a long rest" would be good. Right now it's "Everything is amazeballs. Don't worry about anything."))