So, the guy gets trapped in a mental prison that makes it seem like time is stretched out for him. All the while, his magical pathways are destroyed in his physical body, and he’s stuck in an empty dreamspace where nothing exists (including magic).
So how does it make any sense that he would be able to develop an entirely new magic system, no matter how long he was trapped? He’s in a fake, empty, dream-like construct. At best, he might spend a lot of time theorizing new ways to use magic, but he shouldn’t have been able to test any of it out. The idea that he’s not only found a completely new way to harness magic, but seemingly perfected it is just awful writing.
Plus, you know, years in isolation with literally no additional stimuli would completely drive someone insane, no matter how obsessed they are. But I feel like adding that argument is just nitpicking after the larger flaw.