Rebalancing/Scaling down a Semi-God

One if the big bad's in my campaign is a sorcerer that wanted to be a god. Originally he was going to be more of a late game encounter, but possibly need to change things up due to a chunk of party members dropping without warning. The campaign as it currently is could stop, but the remaining members still want to play. So I considered rearranging the "kill order" for the big bads, and maybe doing a full restart. (Still discussing with my players what they want.) But figured I'd get a jump on it if they're okay with it, and if they want to continue that's set up already.

Anyways, back to the main point. Again he was going to be a later game villain but trying to shift him earlier on. He's also a Sorcerer. More about him is that he's quite narcissistic and has had a god complex for a while.

In my attempt to shift him down, I've considered that instead of ruling the whole Material Plane, he'll still be learning his powers and therefore just has control in a pocket plane. The best way I could thing of would be Wish, and while that worked fine as a high level, not so much at lower ones.

One possibility I considered was due to being semi-god I've thought of doing a split where they are capable of the higher spells, but their power is diluted significantly.

But wanted opinions from more experienced DMs or other possibilities on how I could work around it.

Side note: Sorry for the mini context rant at the beginning.