Before I begin working on this fanfiction, I want to let you know that I'm not going to make him a ridiculously overpowered character. He will have Sun Wukong's powers, but they'll be heavily nerfed to fit the standards of the Jujutsu Kaisen world. Don't get me wrong — he'll still be very strong, but he won't be unbeatable. He'll slowly get stronger over time, and power won't just be handed to him.
Yes, he will eventually gain full mastery of his abilities, but it will be a gradual process. It's kind of a spoiler, but not really, since it'll be revealed in Chapter 1 anyway. God did grant his wish, but Ren never specified that he wanted instant full mastery from the beginning, nor did he mention anything about avoiding nerfs. So yeah… balance is key.
Anyway, I hope you enjoy the fanfic once I start releasing the chapters! And do give me your thoughts since it's my first time writing a fanfic.