I know this might not be an option for you, but I have used Home Assistant for this purpose. Simply install a HA instance and enroll both of your handsets with the companion app.
The challenge with apps like this is, that they need some kind of server by definition.
Newer vehicles will have the inverters too, so taking one apart will have the full infrastructure for a backup power source. Older batteries are already being used for this sort of purpose at least in the Nordics.
The Culture is the absolute best case scenario for humanity. It is my comfort reading for this reason.