Writing the kind of fanfiction you're looking for involves navigating some unofficial spaces, as platforms with strict content policies tend to restrict explicit 'lemon' content. Archive of Our Own, or AO3, is generally the most reliable archive. Its tagging system is your best friend—you can filter for the specific pairing you want, then add tags like 'Explicit' and 'Lemon' to narrow things down. Just be aware that some writers might use more clinical tags like 'Porn With Plot' or 'Smut' instead of the older fandom term.
I'd advise steering clear of the big, algorithm-driven sites like Wattpad for this specific niche. Their content moderation is stricter, and a lot of the truly dedicated smut writers migrated to AO3 years ago. Some older, forum-based archives might still have classic stories, but they can be a pain to search through. Honestly, finding the right combination of character dynamics and steamy scenes is a process of trial and error, but AO3's filters make it less of a slog than it used to be.