Even a D&D campaign at this level is hard to DM, I cant imagine the complexity in trying to really gameify this when especially at the end your players are gods.
Cast wish, may as well just let players write their own code for what they want.
Sure bigger stronger enemies are available but parties become so powerful i can't see it being easy.
I would say just leave those aspects out of the game, or put strict limitations on their usage. You could isolate higher magic abilities to specific items (e.g. deck of many things).
It is complex, however I think if you look to games like Neverwinter as a template for moving forward (absent microtransactions).
I'd actually rather love some sort of different campaign or some adventure packs, similar to how D&D does it with their expansion books.
Either that or a great mod tool that would allow for something like that. Doesn't even need support for added races, classes or traits. Just a map editor with a mob editor slapped on top would be a gamechanger
It does run on steam deck (though at lower settings with FSR to get 40 mostly stable).
Open world games with some complexity generally take a decent amount of power. You have to load a good number of surrounding objects at any given time, with a pretty wide view on the zoomed out view. There are also other characters/animals doing stuff, environmental effects, and a healthy dose of passive checks on the environment against various traits of your party to see if your character identifies any of the secrets all over the world.