Personally, the only thing I need is stable clients on both desktop and mobile with bidirectional sync. Copyparty seems really cool, but it explicitly does not do that.

Have you considered syncthing? There’s shiny new and super cool Sushi Train (or Sync Train by other name) app for iOS (I wish the author would make it a paid app, so much I like it!): https://github.com/pixelspark/sushitrain

Not affiliated, but a very happy user.

I mention iOS, because that was what I needed personally, as there was syncthing for Android since forever.