Google Play Music was the only app to really nail it, but Google sadly put it out to pasture.
For what it's worth, I have had good results with Apple Music. It works great as a "custom jukebox in the sky" for me.
I use iTunes installed from the Windows Store and just drag my own music files onto iTunes. It syncs them to Apple Music where the files stand right alongside the streamed tracks from the paid service. I can put them in playlists together, edit metadata for my own tracks or Apple Music ones, see (and add!) lyrics, and so on. They even have an Android mobile app.
You're dead on, though, that Apple Music is lightyears ahead of Spotify wrt local files.