There's a few projects that have started towards this path with single-click deployable apps, you could even say HomeAssistant OS does this to some extent my managing the services for you.
I believe one of the biggest hurdle for a "self hosting appliance" is resilience to hardware failure. Noone wants to loose decades of family photos or legal documents due to a SSD going bad , or the cat spilling water on their "hosting box". So automated reliable off-site backups and recovery procedures for both data and configs is key.
Databox from BBC / Nottingham University is also a very interesting concept worth looking in to:
A platform for managing secure access to data and enabling authorised third parties to provide the owner authenticated control and accountability.