I gave up downloading when I found out there were many open source yt-music front-ends. I use ViMusic. But to answer your question, I think I would find a torrent or five to download a couple of collections, if I were you. Probably easiest.
Isn’t F-Droid, alternative appstore, just sideloading, made easy?
I use ViMusic, front end for Youtube Music (I think) for Android. Needs to be sideloaded (obviously). You can search up existing playlists. I have not found a single song that can not be found on this app, since it also includes the normal youtube library. It just works. There are other open source front end for spotify that are similar to this, but I’ve been using this for a while, and not found anything bad about it. So highly recommend. Also, the radio function is the best radio function I have ever tested.
This guy has a guide for Rpi5, debian, some disk config, docker, nginx,-proxy-manager, immich. Backup/restore… If you like hands on stuff, this is ok I think…
edit, forgot the actual link: https://github.com/TorATB/RPI-Immich/blob/main/Immich on Raspberry PI5 Guide.md