I don't mind the becoming a girl and losing his powers part, I just hope the author knows how to keep the story engaging given that premise looked like a steamroll MC for some readers.
I for one like it, I don't necessarily hate OP MCs , I also don't hate weak ones, I always loved the stories where the MC starts as weak and progressively goes stronger through conflict and perseverance.
This kinda reminds me of some video games that use this trope, start as OP as fuck, then suddenly makes you start from the bottom again. Given the situation for Dark, this gives (him?) her a goal to pursue and stay in this world for a while.