There is a way of doing this for all rooms in your Sonos system via IFTTT. Just follow these steps (I’ve used the web browser version but it’s possible to use the app as well.)
1/ Log in to IFTTT and then click on your profile picture and select ‘Create’;
2/ Click on the ‘+’ symbol in the phrase, select ‘Google Assistant’ and then select ‘Say a specific phrase’;
3/ Type in your preferred phrase for Google Assistant to respond to. e.g. ‘group sonos’;
4/ Click on the newly located ‘+’ symbol and select ‘Sonos’
5/ Select ‘Play Favorite’ and then select the favorite, a default device and in the ‘Include grouped rooms’, select ‘include all rooms’. Followed by ‘Create Action
6/ Google Assistant will then respond to ‘Hey Google, group Sonos’ and will create a Sonos group containing all of your rooms and start playing the Favorite
If you want to play different music asking Google Assistant to ‘play xx on Sonos’ maintains the Sonos Grouping.
I know this isn’t as flexible as you are all requesting but it does provide some level of grouping by voice to GA.