Home assistant, as a central system (it basically let's you wire anything into anything!). The smart switches etc should be esp8266 or esp32 based. You can then flash either tasmota or esphome to them.

Since your server will likely be Linux based, it's open source all the way to the bare metal, (or at elast as close as possible).

My current system almost doesn't notice if the Internet dies. Also, if you nuke critical components, in the worst case, it still defaults to dumb control behaviour (physical switches still work etc).

I still know where the kill switches are however. I've also made sure it doesn't have control of anything mobile, other than the robo vacs, and I'm fairly sure I could take them in a fight.