Stumbled onto a hidden gem last year that feels almost tailor-made for that vibe. It's not a fanfic site per se, but a personal blog run by an older writer who's been in the EXO fandom since debut. They only post maybe three or four stories a year, all Chanbaek, all incredibly mature and psychological. The prose is dense, more like literary fiction than typical fanfic. I found it through a reblog chain on Tumblr, of all places. The author tags meticulously, so searching 'chanbaek' 'angst' 'rated M' eventually led me there. It's a reminder that the most specific content sometimes lives in the quietest corners.
AO3 is, of course, the overwhelming default, and for good reason. The tagging system is unbeatable for finding exactly the mood and dynamic you want. You can filter for 'Chanbaek (EXO)', 'Explicit', and then add additional tags like 'Domestic', 'Established Relationship', or 'Psychological Trauma' to narrow it down. The quality variance is huge, but the top works are genuinely stunning. I've reread one 150k word alternate universe where Baekhyun is a pianist and Chanyeol a sound engineer about a dozen times. It ruined me for months.
That said, I've found some truly unhinged and brilliant M-rated stuff on older, more chaotic platforms like Asianfanfics. The tagging is a mess, and you have to wade through a lot, but there's a certain rawness to stories there you don't always get on the more polished AO3. Sometimes you want a story that feels like it was written in one feverish, emotional sitting at 3 AM, and AFF still has those in spades.
Honestly, I sometimes have better luck on Twitter (or X, whatever) these days. Writers will post threads, and if a thread gets traction, they often migrate it to a dedicated platform like AO3 later. Following specific hashtags or mutuals who retweet fic previews is a decent discovery method, though it's more ephemeral. You have to catch it while it's hot.