I've been using Fossify Music Player on my Pixel Phone running #GrapheneOS.  My song library is in a folder on my phone that I keep synced with #syncthing.  To add new songs I purchase the MP3's from Amazon using my Desktop #Linux computer, cleanup the MP3 embedded file tags with #EasyTag and then move them to the phone via the shared syncthing folder on my desktop.  I never stream music over a network connection on my phone.