Well, it's Ao3 - that website's full of yaoi fangirls; there's more m/m fics than there are m/f and f/f fics combined. They do have good chemistry though
Also I hadn't heard of that whole "Ningguang is Guizhong's reincarnation" theory; IIRC reincarnation as a whole is never brought up in the story or lore at all but the evidence there is pretty compelling. Even if the theory does turn out to be true, Zhongli probably wouldn't want to interfere or bring it up at all and just leave Ningguang to do her thing as a human.
On a completely different note, I need to remind myself to not read the comments on new (female) characters' showcase videos. Xinyan's video had a lot of people cheering on her joinging Aether's harem, which is just blech