Don’t do it in the post process; just do it in on the sphere object itself and use “ScreenSpaceUVs” instead of the normal TexCoord UVs you’d use on the object.
This creates a screen space “post process” effect that happens only on pixels that the object occupies.
I built this effect a few months ago (I think I’ve gotten it smoother since then); I can send you the .uasset files if you want.
The key thing to remember is that as you enter the surface of one sphere, you come out on the inside surface on the other sphere (so you can’t have two spheres and go two ways); making this effect two way between arbitrary spaces adds a ton of complexity/implausibility.
It looks like you’re mostly interested in spherical microcosms which are actually much simpler 