Since you are looking to install the 16 channel amp what you could do is wire the 8 zones to 8 Chromecast Audio’s. Sure its going to cost a bit but then you can just tell the Google Home app which Chromecast Audio is assigned to which room and then its simply a matter of selecting an audio source to a room. (Nice HA setup here) You could do this by voice if you have Google Home’s around the place to listen in for your commands. I do this on a smaller scale in my house as I only use 1 Chromecast Audio for the lounge and then Google Homes in other areas