bullshit
first, there's the motivation problem. if you spent like, years doing a certain research, you tends to even forget the reason you're doing the research in the first place. common problem among lich and the like
then there's applicability of his power. unless he's laplace, or has access to the akashic record, his imagination won't necessarily has the same rule as real world. it's like dreaming up the perfect perpetual motion machine, only to fail irl because friction exist.
all the more so since he exist in a world where mana cost and exhaustion doesn't exist, like, good job inventing a magic that needs 100 mages worth of mana maintained over the course of 3 days, dumbass
but i'm sure all that won't happen
i'm sure this randos would still hellbent on the destruction of the world even after centuries studying magic, and i'm sure all magic he thought up would be applicable in real life
and here i thought this kind of bull only available to protag