FYI youtube-dl can download playlists. I heavily suspect you can even use --continue to download missing updated bits in a playlist you previously downloaded
Good point on the playlists, it hadn't dawned on me that 'watch later' is a playlist. Haven't tried continue, in the past I've created a 'downloaded.txt' file which gets updated as files download and then checked before it runs again so it doesn't try to re-download them.