Your best bet is lots of screenshots lol.
I’m sure it’s a very complicated process to create a backup for this sort of thing. It’s not like iCloud on your phone where you’re restoring contacts, messages, calendar events, etc.
Now you got secure HomeKit codes to get the device into your account. Then you got zigbee hubs with child devices where the devices only speak to HomeKit via the hub, automations, other family members, etc. Probably a huge mess with a high failure rate that Apple doesn’t want to stand behind. They got enough on their plate with bollocks software lol.