If we're talking strictly original fiction, there's a pretty active corner of AO3 that's way more than just fanfic. People post original taboo romance and dark erotica there all the time, under the Original Work tag. The tagging system is its biggest strength—you can filter for exactly the dynamic you're looking for, whether it's power imbalance, forbidden relationships, or morally ambiguous stuff. The search function is clunky, but once you learn to use tags like 'dead dove: do not eat' or 'morally ambiguous character', you'll unearth some truly wild, beautifully written stories.
A lot of authors who start on there move to publishing on Kindle Unlimited later, so it's kind of a breeding ground for new voices in the genre. The comment culture is surprisingly supportive too, which you don't always get on other platforms. For purely taboo themes, I've found stuff there that even the dedicated erotica sites are too scared to host.
Just be prepared to wade through some poorly tagged fanfic to find the original gems.