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.
For the few YouTube channels where I actually want to watch every single video uploaded by the creator, I do use this. For the rest, I have to hope that they create playlists for the subset of their videos that I am interested in. Many creators are actually quite diligent about organizing their uploads into playlists.
