I actually have a script running on a cron job on my personal server that does approximately this. It downloads into a Syncthing folder that automatically syncs to whatever devices I might want to watch videos on. The difference is I just have a set of playlists for it to download, so I'm relying on the YouTube creators to organize their videos into appropriate playlists, which means it's not exactly "fine-grained". It's basically an 80% solution, and for the remaining 20%, I just invoke youtube-dl manually.