Okay so this is for subsonic, well subsonic is a paid service so i’m not really in favor of that :D But this would check two of my three points. I tried their demo but didn’t find anything for “instant mix / song radio” do you know if it has something like that?
Oh there are tools to download from spotify directly ;) They pretend to be a player, download the songs into the offline cache and decrypt them using your login credentials.
I can DM them if you’re interested. It’s interesting because yt music premium still only givey you 256kbit/s but spotify premium goes up to 320kbit/s