I think I am the luckiest person ever because I have thousands of uploaded music files (all converted to ALAC if they weren't already in MP3) and have never had a problem listening to them on any of my devices. The Music app reads the metadata the files already had and I've been able to import lyrics and the like without an issue. It all even works properly on Apple Music for Android, when I briefly had to carry an Android phone for work.

Plus, I can edit the metadata for tracks that come from Apple Music to my own tastes so I can tag that a song from the service is one version while my copy of it is a different version.

For me, Music is one of those things I hope Apple never "modernizes" because I feel like a lot of this nerd-level functionality would be lost in a major rewrite. It's certainly light years ahead of Spotify's truly pathetic "local files" function.