Mopidy does a great job, either on its own as a player of streams, MP3 files from the local drive or a NAS, or for connecting to various online services. I think you can even integrate shairport-sync to cast to it, but have only got that working on the hifiberry snapcast clients individually.

The only issue I've had with mopidy in general was getting it to work with Spotify. The API changed recently, and doesn't seem to work anymore via raspotify. I set up Tidal instead, and was good to go in ten minutes.

Also, you can send the output to snapcast and have multiple devices in separate rooms playing music synchronously.

There are various web-interfaces, or just use MPD to control the mopidy server.