I wasn’t able to get anything Google to work with Sonos. I originally had Spotify but cancelled when I couldn’t afford a bunch of subscriptions, and switched to my local stores music - which Google also refused to cast easily.

When they removed the ability to listen to my local music library from the app I started looking for alternatives. Seemingly perfectly timed for the app to shit itself.

I found BubbleUPnP. It can cast to all of my Sonos and my Google assistant speaker with zero issues and can play from Plex, my phone’s local library, and my NAS. Highly recommend. Three free version is usable but paid is only $7 and I no longer need the Sonos app.