I ended up in a very annoying situation in which I unlinked my Sonos account from my Google account, but I wasn't able to link it again. The reason why the "unlink" procedure happened, was on request by Sonos Support itself as I was having stability issues with the Google Assistant. They wanted to me to reconfigure everything from scratch.
When setting up the Google Assistant for the first time, there was a step in the Sonos app asking me to allow Sonos to access my Google account. However, after unlinking Sonos and trying to install/configure the Google Assistant again, I never got that question again. This ended up in my Sonos devices telling me Sorry, your Sonos system is not reachable when providing them voice instructions.
Even after having several telephone calls with Sonos Support, we still didn't manage to bring it back. Until I figured out I had to install Google Home next to the Google Assistant application on my iPhone. In the Google Home app, I was able to properly detach Sonos (again) from my Google account. After having done that, I managed to reconfigure everything from scratch.