Comment by ZlibraryKO

I have a setup on GrapheneOS which works using a DNS whitelist So everything is blocked by default (ControlD dns provider) and only a few websites are accessible. The password to the owner profile is locked away and can only be unlocked by waiting, so the DNS setting can't be changed on a whim. I'm very happy with this setup.

This way it does not really matter that you can install apps because to be interesting they need internet access.