No Mutants Allowed

I've found that I like being a player in medieval fantasy but I can't bring myself to DM. My creativity is too heavy in science fiction and fallout/post apoc

Yeah I totally understand that feeling, it's generally why I home-brew so much stuff when I play D&D so that I can make a fantasy setting sufficiently weird enough to interest me.

I tinkered around with a fantasy setting once where the Gods were like Galactus from Marvel, astronauts from a collapsed prior universe imbued with cosmic power and then slapped it on a soulsborne rip off and was just like eh screw it, space opera and Fallout is where I'm at

Imbuing fantasy elements into space opera is more fun to me to DM/worldbuild than straight fantasy itself.

Also if you're going deep end homebrew on the fantasy setting I'd highly recommend the system Worlds Without Number. Way way better than DnD gameplay wise and totally setting neutral.

I think I just saw a Questing Beast video about that system, I stick with 5e just because it's what me and all my friends know but I've heard Numenera is really good. It's basically Book of The New Sun: The Role Playing Game