honestly, the best place depends on what flavor of fic you're after. For the truly dedicated, I've found the most intense, long-form explorations of that dynamic tend to live on Asian fic platforms like Fanfiction.net or dedicated Korean sites, though navigating them can be a language barrier. AO3 is the obvious heavyweight for tagging and filtering—you can drill down into 'canon-divergence' or 'alternate universe' tags to find exactly the mood you want, from angsty slow-burns to more explicit 'hot' content, though their content moderation means some stuff gets a bit coded.
That said, for the real niche, deep-cut stuff that feels authentically 'in' the fandom, smaller Discord servers or Twitter/X circles where fans share Google Docs links are unbeatable. The quality there is hit or miss, but when you hit, it's like finding a secret stash. Tumblr's still good for moodboards and shorter, punchier character studies that feed the ship, even if the full stories often link out elsewhere. My personal library is a messy mix of bookmarks from all of them.
Honestly, I'd suggest starting with AO3 for volume and searchability, then branching out if you're craving that unfiltered, community-specific vibe. You end up curating your own little archive after a while.