Oh, that stinks. I’d love to be able to publish it somewhere I can get genuine feedback. I’m honing my writing skills again in preparation to pick up an old novel I wrote years ago.

Even better! Writing, and reading other's writing, is the way to do it; I feel like my writing skills improved massively after my first fic (hence, I'm trying to get better prose in the second).

I publish on Ao3 too, but after a while I also upload a few chapters at a time to Fanfiction.net. As janky as it is, it still has a lot of readers.

I dunno where else you should consider publishing... Tumblr? Maybe anime forums? A Fediverse animation group? There might be places with a large One Piece following, including oldschool forums where posting fics is a thing. But I can tell you (from my experience with fandoms) Reddit subs are not it. I dunno what it is about the format, but more speculative lore discussion and fanfics just don't have much traction there.