Weirdly enough, I've had the most consistent luck on Archive of Our Own. The tagging system is a lifesaver for a show like 'Midnight, Texas' where the fandom isn't huge; you can really drill down into specific characters. I usually filter by Lemuel and Olivia, because their dynamic is just endlessly fascinating to me—all that ancient, weary power mixed with a sharp modern edge. The quality varies, but I've bookmarked a few authors who nail Manfred's voice, that mix of vulnerability and stubbornness.
I'd steer clear of Wattpad for this one, honestly. The few times I looked, it was mostly abandoned reader-insert stuff or crossovers that didn't really grasp the show's Southern Gothic tone. Tumblr can be a good hunting ground for shorter drabbles and character studies, especially if you're into the more philosophical side of Bobo or Fiji's witchcraft. You just have to be willing to dig through a lot of reblog chains and gif sets to find the text posts.