We haven't used the Familiar Faces option, but for us, we can only enable it for our existing Nest Hello doorbells and not for our existing indoor Google Nest camera; that camera just isn't listed under Familiar Faces.
I found the Google Nest Hello page below, but it doesn't seem to specifically list the existing indoor Google Nest camera:
Sorry for my late response. Thank you for all of your patience while waiting for a reply. I can definitely understand how frustrating your situation must be. I just wanted to make sure that your issue is addressed.
Please let me know if you need further assistance.