Neon Genesis Evangelion is kind of like this? Not so much the MC but with a certain red haired side character.

Earthsea spoiler:

!Not manhwa/manga/anime, but Ged from Earthsea loses his magical power and need to reinvent himself as a regular guy!<

There's a few in that vein or similar. Specially korean mangas like https://myanimelist.net/manga/150210/After_Ten_Millennia_in_Hell

And some fantasy japanese ones where they reincarnate and are nerfed or were OP in a game world got reincarnated and the skills are nerfed, so the whole thing is more about seeing how they grow to get big again.

But I guess these ones are still in the same vein as power fantasy because it's about seeing them grow into being super OP again. So not exactly what you want I guess.

I can't remember the title, but there was one manga about a demon lord that decided to reincarnate as an average person, but the average person in his era was really powerful compared to the people of the era he was reborn to, so it defeats the purpose since he was still considered OP despite the big nerf.