Bit late as a reply, but I remembered your post when I came across this fully open source MP3 player. It does not even appear to have BT, so might be an option.

As that might be relevant, I came across it from this youtube video here (got it in my feed, haven’t watched it yet).

Edit: nevermind, has wifi and bt after all. Not surprising as it’s ESP32 based, so it’s literally just there.

Nextcloud can’t do two-way sync on Android. At all. That’s like core functionality for the product IMHO and there’s a feature request open I think. When I found that out, I basically spit out my coffee. It’s fine if you just want to upload photos you take, that kinda works (but my god is it fragile).

Nextcloud is pretty good at quite a few things, including extensibility, but having some omissions in functionality that boggle the mind.