> Is this a thing that I don't know about? Or is the market too narrow to be useful? (Otherwise, why doesn't it already exist?)
I'm not super familiar with it, but I think what you describe was/is the goal of Sandstorm (https://sandstorm.org/).
Then there are also efforts like YunoHost (https://yunohost.org/) which are kind of like that, gives SSO auth and everything out of the box for all the apps it supports.
Even easier to use and less involved would be maybe what TrueNAS Core has in terms of apps support, which is essentially also "one-click install" of self-hosted applications, backed by local Kubernetes installation if I remember correctly.
A more involved option for people who want to manage more themselves (both infrastructure and configuration-wise) is using NixOS, which is the approach I chose for my own local infrastructure at home. For the packages supported by NixPkgs, many applications are like ~4 lines of configuration to setup and get integrated with the rest of the apps you run.
All of these options you can run in the cloud, bare-metal or at home servers, afaik.