On Volumio 3.179
I can somehow not believe this is not implemented. Was thinking about posting it in the Bug section of the forum. Also did not find a feature request for it as well (maybe I missed it).
When I listen to Spotify or Qobuz (no Tidal account) on Volumio and I add a song as a favorite, this should be synced with the corresponding list on the provider side. Of course only Spotify Favorite to Spotify and Qobuz to Qobuz as the Volumio Favorites is a global list covering different sources.
Also Playlist support/sync and the ability to add whole albums to a playlist needs major improvement, but here I already found topics covering the issue.