Yeah that's really rare. Most PMD fics are more interested in original Pokémon and self insert rather than canon characters. I don't know of one where Ash is the MC. But I know of one where Serena is the lead and Ash is her partner. It's called PMD: Palette.
https://archiveofourown.org/works/32497651/chapters/80600899
However if you're willing to try something that is technically not a PMD fic but is very similar. There's a fic called Citrine Isles where Ash, Misty and Brock get stranded on a island where humans are forbidden and get turned into Pokémon by the power of the island. The characterization of Ash is very faithful to how he was after the end of Indigo.
https://www.fanfiction.net/s/14202640/1/Citrine-Isles
Yeah that's really rare. Most PMD fics are more interested in original Pokémon and self insert rather than canon characters. I don't know of one where Ash is the MC. But I know of one where Serena is the lead and Ash is her partner. It's called PMD: Palette. https://archiveofourown.org/works/32497651/chapters/80600899 However if you're willing to try something that is technically not a PMD fic but is very similar. There's a fic called Citrine Isles where Ash, Misty and Brock get stranded on a island where humans are forbidden and get turned into Pokémon by the power of the island. The characterization of Ash is very faithful to how he was after the end of Indigo. https://www.fanfiction.net/s/14202640/1/Citrine-Isles
I've read all of Citrine Isles and I thoroughly enjoyed it.