Every time I remove a song from a playlist on PC, then sync to iPhone via iTunes, the playlist is updated (i.e. it doesn't include the removed song), but the song is still in the library. That adds up over time. Now a bunch of "rogue" songs have accumulated that aren't in any playlists that take up quite a bit of storage. Any way to get rid of them?
How about only syncing the playlists you want instead of the entire library in iTunes (or Finder if your computer is macOS based)?
If you are someone that learned to drive a car without an automatic transmission, your method is akin to manual shifting. You could also enable Apple Music or iTunes Match and then you no longer have to micro-manage which I played songs get removed as your device keeps track and cleans automatically when you run low on free space on device.
Set up a smart playlist that has all the songs that aren't in any existing playlists. Rules:
Then drag the playlist onto the iPhone, select all the songs in it and press shift-delete (right-click ->
delete from library didn't seem to work).
That's still pretty cumbersome (having to add every playlist to the smart playlist manually), so I'm open for other answers. However, this is faster and less error-prone than removing one-by-one.