Simplest is to use syncthing and just sync everything to your phone but this won’t cover a lot of your use cases and is probably best for a one user experience.
Lidarr for new music + a subsonic server such as gonic will cover a lot of what you need. The idea is to find and download music(lidarr+dl client) and run your own streaming server(gonic or other implementations). On mobile you use an app which supports the subsonic protocol (such as substreamer or tempo) too listen. You can also just use jellyfin server + it’s client, but AFAIK, the music experience is not as good.
Imo too high power consumption for 24/7 operation so I wouldn’t use it that way. If I only had this machine to work with, i’d probably use it as a media server or NAS but turn it on only as needed. Wake on LAN to turn it on and configure it to auto turn off.