I’m 99% sure Google won’t allow selfsigned, you can get around it locally by adding the CA in your cert store, but you can’t add that same cert on Google’s servers. I’m not sure which platform you are running HASS on, but if you have access to a docker host, you could use something like this: https://github.com/linuxserver/docker-letsencrypt
That would probably be the easiest way to set it up, as cert creation and such is totally automated.
Also if you set it up for remote access, you may find use cases for having remote access (We use it at home to signal trying to get hold of the person at home, if the person does not pick up the phone, as we often put them down somewhere when at home).
I’m just happy that Google actually trust the Let’s Encrypt cert, as we would otherwise have to pay for a cert.