Airfoil [1] is worth every penny. The same could be said for pretty much everything Rogue Amoeba makes.

It’s also worth noting that Spotify can act as a remote for its own service across devices. So you can start a song playing on your phone and direct the output over AirPlay and then just interface with that session using the desktop client.

[1] https://rogueamoeba.com/airfoil/mac/