No one's mentioned Ficwad yet, which is a real shame because it's got a solid chunk of the good older stuff that hasn't migrated to bigger sites. A lot of Ash/Dawn, or Pearlshipping as we called it back in the day, got its start on forums and smaller archives. Ficwad still hosts a bunch of those classics. The search function is a bit clunky, but sorting by favorites for the 'Pokemon' fandom usually surfaces some real gems from like 2008-2012. The writing style there feels different—less polished sometimes, but there's a genuine enthusiasm that captures the spirit of the original series era.
Honestly, the real treasure hunt is on DeviantArt, but it's a mess to navigate. You have to dig through groups like 'Pokemon-Fanfiction' or 'Pearlshipping-Fans' and sift through a lot of art and memes to find the prose. Some authors cross-post, but a lot of the best short stories and one-shots live only there. It's worth the effort because the mood pieces you find are unique—more atmospheric, less plot-driven than what you typically get on AO3. I found this one author who writes these quiet, post-Journey reunion vignettes that absolutely nail Dawn's perspective, something I rarely see elsewhere.
If you're after long-form stuff, FanFiction.net is still the powerhouse despite the purges. Filter by Ash/Dawn as the pair, sort by reviews, and you'll hit the big ones like 'The One Who Stays' and 'Diamond in the Rough.' The community around those stories is half the fun; the reviews sections on those epics are like time capsules of the fandom's reactions chapter by chapter over years. Just be prepared for some… dated tropes and the occasional abrupt, abandoned story.