I updated my backup router without any issues at all, nothing special, just chose the update from console option.
Other than spending way more time than I should have trying to figure out how to make it work without conflicting with the identical configuration on the main router. I usually just plug it into the 5G router to do that, but Three were having a moment.
I was completely unaware how easy it is to share your Internet connection on Linux these days. I just added my on-board NIC to Network Manager (its normally not in use as I have a 10G PCIe NIC) with Method "Shared to other computers" and bam, can use that as the WAN interface with the NAT hiding the main router.
Honestly, half the fight was I constantly kept forgetting I have to connect to the router on the LAN VLAN. I probably wasted an hour wondering why DHCP wasn't working. Then the config file I exported from my main router was corrupt, something is off when you backup RRD data.