There are going to be challenges as they will all come up with the same IP address, so you’d need to either to sequentially connect to the wireless SSIDs that are seen (assuming that they are all different), or do it wired with a managed switch that “activates” one at a time.

Something as simple as

while there_are_still_routers_to_configure do
    activate_next_router_through_switch_cli
    scp path/to/image.bin root@192.168.8.1:/tmp/
    ssh root@192.168.8.1 sysupgrade -n /tmp/image.bin
done