Some support acting as a client for Apple Music, including Marvis. The API is quite comprehensive, they can do most stuff the official app can do, including libary curation and discovery stuff. One exception is downloading songs in advance (you can add new music to your library, but not download it to your device; however if you go back to the Apple app to kick off the downloads, the downloaded songs are played through the 3rd party app after that). If you just add to your library and stream, you wouldn't need to do that.