Romance-focused Dimension 20 fanfiction? I’ve been searching for this myself, and honestly, it’s a bit of a scattered hunt. The main hub is definitely Archive of Our Own; there are already some solid collections tagged for specific seasons like 'The Unsleeping City' or 'Fantasy High.' The pairing tags are pretty thorough, so if you’re into, say, Riz and Gorgug or Adaine and Aelwyn, you can filter for that. It’s also the best place to find the longer, slow-burn stuff that really digs into character dynamics.
But don’t sleep on Tumblr. A lot of the fanart and headcanons that inspire the written fic live there, and sometimes writers will post drabbles or snippets directly on their blogs before cross-posting to AO3. The tags #dimension20 and #d20fanfiction are good starting points. The community feels a bit more immediate and conversational there, which is great for finding writers who are currently hyperfixated on a specific pairing.
I’ve seen a handful on FanFiction.net, but the tagging system is so antiquated it’s almost not worth the effort. You really have to know the character names as they’re spelled in the fandom and dig manually. Wattpad has some, but the quality can be wildly inconsistent. My advice is to set up an AO3 account, subscribe to the tags for your favorite ships, and let the kudos and bookmarks guide you. The fandom is still growing, so new stuff pops up regularly, especially after a new season drops.