The google app would have to provide that hook to the os, so u til that happens there’s nothing you can do easily.
I am backdooring in to the nest via the ifttt api because the nest app doesn’t provide anything to shortcuts, so I can’t use anything in the local app.
I have one That works without tapping anything. There was a setting in Assistant to ensure it was going to answer to voice alone.
I ask Siri to “hand me off to google” then I’m talking to my usual google assistant! See here as I asked Siri for Google then told google I was just testing…