Schlatt-centric fic is tricky, honestly. He's more a YouTube personality than a fandom constant, so the real depth is on sites that embrace that niche. Tumblr has some dedicated blogs, but the tag is a mix of memes and text posts; you need to dig through reblogs to find the actual stories. Archive of Our Own has the most structured content—tagging system lets you filter for 'JSchlatt/Reader,' and the quality varies wildly. I've found a few authors there who really get his chaotic-yet-charming voice, writing scenarios that feel true to his streams without being overly saccharine.
Discord servers around his community sometimes have fanfic channels, but those are hit-or-miss and often unedited. Your best bet is to search Ao3 by bookmarks or kudos, then check the authors' profiles—they often link to their Tumblr or Carrd sites for more. Don't expect epic novel-length stuff; most are one-shots or short series, but that fits the vibe. The real gems are the ones that balance humor with the soft moments, like a fic I read where the reader works at a failing coffee shop he keeps 'reviewing'—it captured his persona perfectly.