Did you click my snapcast link? It natively supports Spotify connect and Airplay, so you'd connect your mixer to the Pi's 3.5mm jack (or use a supported DAC).

Did you click my snapcast link? It natively supports Spotify connect and Airplay, so you'd connect your mixer to the Pi's 3.5mm jack (or use a supported DAC). You can also fashion a Chromecast audio out of a normal (video) Chromecast and an HDMI->audio adapter if you're looking to preserve the same experience.