I am far from proficient with all of this, but I had an idea that I thought may be worth sharing. When I try to cast to google devices via Spotify on my desktop, I cannot see any of the google devices. I can only see the desktop I am on and a Firestick. The issue is referenced here (https://community.spotify.com/t5/Desktop-Windows/Spotify-Connect-amp-Google-Home/td-p/4642461) and I can confirm that I see all of my devices on my phone, but not from my desktop.
Maybe the limitation is with the Spotify API or the way that Home Assistant is using it, because it functions very similarly to the way that using Spotify on my desktop does. If I start playing music on a google home (via voice command or casting from my phone), I can see the google home on my desktop Spotify client in the devices menu but the moment I switch it to my desktop via the devices menu on my desktop the google home disappears. This is the exact same way that sources show up on my HA front-end and it behaves the same if I switch devices from the HA front-end.
Again, I have no idea how to begin remedying this, but I figured I would throw it out there in case someone who has the knowledge wants to take it and run with it at some point. Maybe there is a way to utilize the iOS or Android API to see google devices with more reliability.
Cheers