Archive of Our Own is overwhelmingly the hub for it, honestly. The tagging system is a godsend for a fandom with thirteen distinct dateable characters, plus side pairings and poly dynamics. You can filter by specific brother, by genre, by kink—everything. I’ve found some incredibly nuanced character studies there that you just don’t see on more generic sites. Some authors really dig into the demon lore or the celestial realm politics, which adds a layer of depth that the mobile game can only hint at.
Tumblr still has a vibrant scene, but it’s more fragmented. The best stuff often gets cross-posted to AO3 eventually, but I’ve stumbled upon amazing WIPs and headcanon threads there that feel more immediate and conversational. The downside is that if you don’t know the right blogs to follow, it’s easy to miss gems. Wattpad has a massive quantity, but the quality is... wildly inconsistent. It’s where a lot of new writers start, so you get a lot of trope-heavy, first-person reader inserts that can be fun in a fast-food sort of way, but rarely groundbreaking. My personal strategy is to use AO3 as my primary library, Tumblr for real-time fandom chatter and mood boards that inspire fics, and I’ll occasionally dip into Wattpad if I’m in the mood for something very specific and self-indulgent, like a high school AU.