Mailinabox [1] almost does it. They target Ubuntu stable, and upgrades are mostly silent. But they needed user intervention when Ubuntu had to be upgraded from 18.10 to 22.04, which was quite challenging.
If sandstorm can get there, then putting it into a RPi box is simple. What's difficult is helping the general public understand the importance of protecting their data, so such a product has a reasonable market.
There are probably a couple interesting approaches. One might be to have users point some other domain at a Sandstorm API endpoint, end-running around Sandstorm's own domain, which might work in scenarios remote servers are only fetching. But another idea would be to sort of "special-case" an ActivityPub server deeply integrated into Tempest, and instead of letting a user install "Mastodon", they are installing a custom frontend that creates a user account on the server's ActivityPub.
Because Sandstorm was intended to work as a large scale hosting service, it avoided pretty much any design which could let one user know about another user on the server, but Tempest may end up making choices more suited to smaller deployments where we assume a Tempest server hosts either one organization or one family or one group of friends, etc.