OpenSource-Ghost Every app can support unattended updates. They can do fully automatic updates for apps where the user authorized them to install the currently installed version. Please read https://grapheneos.org/usage#sandboxed-google-play because you seem to be significantly misunderstanding the sandboxed Google Play feature. It has no special access or privileges. You're implying that it does which is wrong. Our compatibility layer uses the standard unattended update support for the Play Store.
F-Droid and Aurora Store are both very outdated and don't support this. F-Droid is missing support for using the modern Android package installation system and many other features. It has a lot of missing security features and doesn't follow the intended app source model correctly. Enabling an app source is not meant to enable installing from more than one source and this is incompatible with it being a trusted app source. F-Droid privileged extension breaks multiple standard OS security features and is also completely unnecessary for unattended updates if they simply did things properly.