I think a smart home hub of sorts should be something you can install in any home, like a water heater or an internet router. The hub connects to your wifi and manages all of your smart devices. You register your devices with it using a web interface that is only just like configuring a router

The web interface part would be the really complex piece I think. You would have to have some way for devices to tell it what features they have, so it could dynamically produce the appropriate controls per device

I dunno if this is really feasible to actually build but it's one of the only ways I can think that would allow average non-technical people to do this without having to administrate their own hardware and network settings and such

You have described Home Assistant. That’s exactly how it works, including offline if you need it and auto discovery of devices on the network.