Right, but as you said, if you connect to the LAN port then you can only have one device plugged in at a time… all I was thinking was if you did it using the WAN ports, by default they are configured to be DHCP clients… so assuming your “master flashing device” with the target firmware files available to it could also run a DHCP server, then in theory you could connect and flash as many units as you wanted without worrying about IP conflicts/etc.

For example, assuming SSH is available by default on the WAN port, my take on this would be:
a) Configure DHCP server to execute a script anytime a new DHCP lease is handed out.
b) Said script would check the MAC, make sure it matches a GL-Inet device range
c) wait a specific period of time (or do a ping loop waiting for a response) to give the device a chance to boot fully
d) Use SCP to connect to the device and pull the os_release or some other custom file that would allow you to check and see if device is running proper custom firmware or still at factory stock.
e) If still factory stock, then issue the SCP and sysupgrade commands as you mentioned before.

That way the process would be 100% automated. Literally all you would have to do is plug a bunch of units in and wait until you see them reboot on their own… or be really lazy and just leave them for a long enough time frame for things to complete (say 5 minutes), then unplug them all and do the next batch etc.

Just adding to the thought process as that’s how I’d tackle it, YMMV