An article was posted recently that you can host the Firefox sync server yourself. Supposedly a bit fiddly, but doable. If you’re considering self hosting something anyway, might as well be the right tool for the job.

Hope these help:

• https://blog.diego.dev/posts/firefox-sync-server/
• https://github.com/mozilla-services/syncstorage-rs

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.