First off, you're a kickass parent, that's an awesome thing to do for your daughter. How easy is it to sync with music she wants to listen to? I haven't touched Rockbox in quite a while, but I imagine it's still expecting DRM-free audio files of some sort right? Other than bandcamp I can't think of any good source for finding some beyond ripping audio streams and organizing it yourself.
A lot of my drm-free mp3s come from friends. I probably have 2000+ cds that I’ve ripped, or downloads from buying vinyl. I’ll make a copy for friends to load onto devices and it’s the equivalent of loaning a disc to a friend.
I suppose there’s probably music torrent sites out there, but haven’t heard of anyone pirating music in many years as I’m too old now.
Amazon Music sells DRM-free MP3 files.
Qobuz too!