i plan to get a similar setup (music on homeserver, synced to phone for offline use) but i dont need to sync playlists as i rarely use them, i have a streaming account with one(!) playlist with all the songs i remembered and wanted to listen to but didn’t buy as CD back then and use the radio like streaming options a lot.
but for syncing phone with nextcloud i use FolderSync (Pro) and it works as it should. it has lots of possible sync targets and lots of options to sync one or both ways. i have folders with >8000 files that take some time to sync but it works fine in the background with no prob, i let it sync over mobile network too, cz i value a more reliable in-sync status more than bandwidth. however i didn’t really try “immediate sync” for new/changed files yet as i don’t see the need for this but its one of many options.
however i only use nextcloud sync in one or two-way syncs and once used sftp for a one-way sync, so i cannot judge all the other options, but if your playlists are organized in files, their two-way sync might be as easy as with the songs. i bought the pro version on their website so my license is not bound to a google account.
not the best painting ever, he’s partly behind the left car besides the lady, but still clearly vin d.
update: oh damn, looks like i wasn’t able to post the pic too, so my answer has to be “no”, i obviously can’t do that 😥