I have an old small form factor PC powered off in the rack beside my main server. It has an old HA image ready to be upgraded and have the latest backup restored to it.
If the main server hardware fails for whatever reason it’s a matter of 10-20 minutes at most to be up and running again. My zigbee coordinator is connected via serial over Ethernet so that does not even have to be moved.
Everything in the house can be operated manually, lights, alarm, irrigation, media players, etc… during this time. All that is missing is automation. Which while nice, isn’t really “life support critical” in nature.