I'd really appreciate this feature, mostly so I can keep my LB playlists synced with the shared playlists I have on Spotify with people that will stay there. Losing that social aspect is the worst part of moving out of Spotify for me; this feature would allow us to maintain that.

It'd be cool to have the option to choose between "Replace songs" and "Add songs":

"Replace songs" would make exact copies from source to destination playlist (so removing a song in the source playlist would also remove it in the destination playlist).

"Add songs" would just add songs from source to destination, never deleting anything from destination, ideally skipping songs that are there already, to avoid duplicates.

Soundiiz handles their sync features pretty well imo, that could work as a reference