You said you don't mind novels!! Well i've got news for you my friend, how you can go through my list of reincarnation bl novels!!!
https://www.novelupdates.com/viewlist/97854/
And this list right here!!!
https://www.mangago.zone/home/mangalist/2810149/
None of this have dying /post-apocalyptic world tho and none have an mc who is imperative to the world building because of his divine power.
But thank you for the suggestions anyway
I read the first half saying you wanted reinc! And thought you said "or" instead of "but" I got confused i'm sorry ╥﹏╥
I don't care it it's a novel or manhwa/manhua/manga, I am looking for a BL where the MC is transported to another world or even originally from there, but the world is dying, or post-apocalyptic and he has a restorative power of sorts and like the last hope for the remaining humans to survive. Or maybe even heal the world.
Like the MC from Silver Diamonds or Uraboku