I don't see why you can't flip a giant switch to go from grid to home power. It's not automatic, but this tech is well established for gas generator cutover in factories and remote homes.
The most common solution to that is a switch that goes through fully disconnected in the middle, and I've even seen a sliding metal plate used that interferes with at least one or both circuit breakers.
In the US they cost a lot more, but they still exist; there's a 750-watt unit at Horror Fright for US$70: https://archive.fo/1pzdK