Man, navigating the Genshin fic landscape for specific ships feels like trying to find a specific ruin guard in Dragonspine sometimes. For Zhongli/Aether, I'd say your main hubs are definitely AO3 and to a lesser extent, fanfiction.net. On AO3, the tag is usually 'Zhongli & Aether' for platonic stuff and 'Zhongli/Aether' for romantic, though sometimes people mix them up—always double-check. The best way to find the popular ones is to sort by kudos or hits; there's this one slow-burn called 'Contractual Obligations' that's pretty much legendary in the corner of the fandom I lurk in. It nails Zhongli's formal, ancient voice and Aether's weary traveler vibe.
Don't sleep on Tumblr either, honestly. A lot of writers cross-post snippets or threadfics there, and the reblog chains can lead you to some amazing, under-the-radar stories that might not have massive kudos counts on AO3 but have huge engagement in the community. The tagging system there is chaotic, but searching 'zhongther' or 'morax traveler' sometimes pulls up gold. My feed is basically 40% Genshin screenshots, 40% fanfic recs, and 20% people arguing about Paimon now.
A weird tip: sometimes the best fics aren't even the most kudo'd ones. I found a fantastic, moody character study by filtering for completed works and sorting by date updated; it had like 200 kudos but the prose was so much sharper than some of the top hits. The algorithm favors longer, chaptered fics, but some of those one-shots with perfect bittersweet endings are what I keep going back to. My bookmark list is a mess of 'to-read' and things I've reread three times already.