There may be an easier way to do it, but I went with the burn-it-down-&-start-from-scratch approach…
I don’t actually integrate much from my ST setup into GH, so this wasn’t too painful for me.
In the GH app, I unlinked my ST account altogether. I think that action removed the “Google Asistant” smartapp from within my ST app, but maybe I then manually removed that too - I don’t recall for sure.
I then re-linked my ST account in the GH app, just as if it was a new setup.
Once that was done, the Google Asistant smart app automatically reappeared within the Smartapps section my ST app - in this new form, it’s now named just plain “Google”.
Within the ST app, tap on that new “Google” smartapp - that will open the various setup management options…
Start by DEselecting (i.e. turning off) the first option - “Allow Google Assistant to access all devices and routines”…
When you do that, then you get the option to select which specific Switches and Thermostats within ST that you want added into GH. Routines are still an all-or-nothing deal.
Save changes - be sure to tap on Done in the upper right corner.
Then go back in the GH app, and confirm that your desired selections show up there… Finally, assign nicknames and rooms as desired.