Look, I've been in the EraserMic corner of the fandom since before it had a proper tag, so I've seen the landscape shift. The best stuff isn't on a single site anymore, it's about matching your mood to the platform. AO3 is the undisputed king for quality and tagging, hands down. You can filter for exactly the dynamic you want—established relationship, pre-relationship, angsty pining, the fluffiest domesticity. The sheer volume of talented writers who treat these characters with such care is staggering.
That said, I have a massive soft spot for the old, rambling multi-chapter fics you'd find on FanFiction.net back in the day. The site's search is a nightmare, but there's a certain charm to those older stories written before the anime fleshed out their canon dynamic. They have a rawness, a guesswork quality that's fascinating. You have to dig, though, and be willing to wade through some rough prose.
Honestly, don't sleep on Tumblr either. Some of the best drabbles, headcanons, and shorter, more experimental pieces live there, buried in tags and reblog chains. It's less about complete stories and more about evocative snippets that spark your imagination. My favorite ficlet about them just existing in the same space after a long day, with Mic humming while Aizawa grades papers, came from a Tumblr thread.
TikTok and Twitter are for recommendations and vibe-sharing, not really for hosting lengthy prose. That's where I find links to the good stuff on AO3. The community there is fantastic for shouting about that one perfect line that wrecked you.