You can call Post-Crisis Superman is a universe buster or whatever you like, I don't want to put a character( Especially like Superman those have many appearances and many different writers) in a specially levels since different writers make his power levels varying, My arguing is Superman did it( Smashing infinite realities, Surviving a sun-eater explosion) or didn't( He smashing infinite realities just a mental thing, Or he not surviving a sun-eater explosion etc)
And is "an insubstantial spirit" not a merely thought and I already posted souls = energy, Astral plane also is energy in dematteis cosmology
Superman smashing infinite realities not just a mental thing, Because he need enough energy(Himself) to smash infinite realities, So you can say it not physically, But certainly not just a mental thing
"i'm actually experiencing another plane of reality!disease doesn't just happen on the physically level...it begins in the aura....the energy field....and then moves down the doy from there...and these...."superman" are thought-forms of the astral world!"
https://i.imgur.com/Z3TpJd2.jpg
"you're a discarnate soul,a thing of spirit,energy"
https://i.imgur.com/4YGtzWO.jpg
"as a green lantern you learned all about the uses of will,didn't you?how,in concert with imagination,it can harness energy--and manifest miracles"
"then i free you from your self-imposed limitations"
https://i.imgur.com/4YGtzWO.jpg
https://i.imgur.com/73onWvz.jpg
"you see,existence itself is nothing more than energy"
https://i.imgur.com/fCX9Mng.jpg
Besides, Superman his body containning the energy which could smash infintie realities, So certainly his physical form also should be that strong