Ah, I didn't see the advanced audio documentation until your message.
Thank you very much, Kingosticks! I really appreciate your help.
On macOS I've also had to add a tee gstreamer element to get the default audio output (thanks to this post). For anyone else finding this, I used:
output = tee name=t t. ! queue ! autoaudiosink t. ! queue ! audioresample ! audioconvert ! audio/x-raw,rate=44100,channels=2,format=S16LE ! $
Both the ncmpcpp visualizer and this cli-visualizer now work with Mopidy-Spotify on macOS.