Ozonelayer
Idk if your taking suggestions but could you make kcriptainite actually rare because in a lot of fic and even the cartoons it’s like everyone and their mother has some form of it
If I was Kryptonian in Marvel, the first thing I would do is borrow the Time stone. I'd go to a Blue star and freeze the time of the universe outside of the star and say inside for like a million years and then go to a quasar and make that power my base power. Quasars make whatever strength and power you get from other stars after going to one, permanent. So even if he gets hit by kryptonite, he'll just weaken to that level of power
In fact, Superman One Million has the powers of a type of force of creation and life that is the most powerful in the DC universe and still has the powers of all Supermans, including the dwarves of the 5th dimension whose strength is only inferior to the presence of DC, which would be the omnipotent being the God of DC. That said, I think it would be cool if the end of this story was the MC ended up uniting the entire Marvel multiverse into just one, like creating a justice league tower but with portals to all the worlds in multiverse