First place I'd check is Kindle Unlimited if you already have a subscription. A ton of authors publish there exclusively, and you can filter for romance and then sort by steam level. Authors like Tessa Bailey or Jessa Kane have some seriously spicy stuff bundled with real emotional arcs—Kane's novellas are short but pack a genuine 'falling hard' vibe into like eighty pages. The 'romantic plot' part depends on your threshold, though; some readers want full-on epic sagas, while others are satisfied with intense emotional commitment developing alongside the physical stuff.
Don't sleep on the 'Books' section of Archive of Our Own either. It's not just fanfic anymore; loads of writers post original works there under Creative Commons licenses. The tagging system is a godsend for finding exactly the dynamic you want—'slow burn,' 'emotional intimacy,' 'established relationship'—and you can filter for complete works and word count. I've stumbled upon some original vampire or fantasy romance series there with incredibly developed world-building and relationships that would put some trad-pubbed books to shame. The key is using the right tags and being willing to sift.
Public libraries through Libby or Hoopla are another goldmine. They've massively expanded their digital collections of indie romance, and you can borrow popular titles from authors like Katee Robert or Kresley Cole. The waitlists can be long for the hottest titles, but placing a hold is free, and you often discover similar recommendations while you wait. It feels less risky than some random website, and you're supporting authors through library licensing. Honestly, my TBR is mostly holds from Libby at this point, and the 'spice with feels' category is surprisingly well-stocked.