In my case, a self hosted streaming server works wonders. Plex with Pleaxamp, Jellyfin, Navidrome, Airsonic, any of them will stream to your phone while out and about.
I live in the rural midwest with spotty cell service. All of those services support manual offline syncing to store music on your phone. I set Plexamp to stream lossy over cellular, and it doesn't take long to cache an entire playlist when I do have a signal.
And I said offline syncing a playlist (1-2Gb), not 400Gb. Max 1-2Gb. I have 1.8Tb of music that I can stream in the rural midwest at any given moment providing I have a signal, and about 3Gb synced to my phone when I don't have a signal. Plex is smart enough to buffer the next few song in your playback queue so it will play seamlessly through bad cellular coverage. Spotify and Tidal work the same way. Is selecting a subset of Spotify's catalog annoying?