You need YouTube Premium to be able to download videos to watch offline (plus see no ads)
As also mentioned by others, youtube-dl seems kinda dead. However there is a good fork/successor called "yt-dlp" which, in addition to other nice improvements, also somehow manages to work around the enforced heavy bandwidth limit by YT.
I haven't had it running in about a year though, couldn't be bothered after a move.
Jellyfin looks nice. I'll have to give it a go. I am dreading having to set everything up again if I move off of plex, rather than just grab my docker-compose file and get going.