can one stream a mix of music sourced elsewhere and bandcamp music? If people have to keep switching apps, I think that's going to limit the appeal to people who are a bit more serious/intentional about every "listen". There's a lot of those, but it's still a minority.

While not automatic, YouTube Music can do this. You can basically upload your own music. Create playlists that mix yt songs and songs in your collection. You can upload on one machine, and access the uploaded song from anywhere, so you're basically using their cloud.

Wait, really? Won't I get accused of pirating my CDs? Is it officially okay to rip my CDs to a cloud provider?

I can tell you that I wasn't accused, but that it's also for private consumption only, no way to share the songs, so they might not be that strict. No idea though. You can read about the feature here: https://support.google.com/youtubemusic/answer/9716522

You can deploy navidrome, point it at a hard drive full of your music, and then stream from the frontend or using an app like subtracks.

Any time I download music from Bandcamp, I drag it into Apple Music on my Mac and it syncs to all of my devices, sitting in my library like any other album.

It’s actually one of the reasons I use Apple Music over Spotify, the process to do this in Spotify is much more convoluted.

Spotify’s process is that the files need to be locally accessible from any computer playing the music, or they must be synced manually to the mobile app.

Lack of a real library is why I ditched Spotify for Rdio years and years ago, and why I switched to Apple Music when that became a thing. And I still run a Plex server, partially for the PlexAmp player.

But you still can’t even just add them to your library in Spotify, you have to put them in playlists, and then download that playlist on your phone, and then they’ll be on your phone. At least that was the system last I used it