I thought I’d try connecting the unit to the upstream 2.4 GHz network to see if that shows any different behaviour, but haven’t gotten that far yet, and found

** Slate router UI doesn’t accurately reflect upstream WLAN connectivity status, in either error messages or not
** Deleting a wireless network from the Internet > Repeater > Join WLAN menu does not prevent the unit from connecting to the wireless network after a reboot

So I’ve added them to the problem description above. Screenshots below.

I deleted the upstream 5 GHz network, then rebooted the unit. I see Repeater (STA) is disabled, but, the unit is still connected to the upstream 5 GHz network, and is still not broadcasting its own 5 GHz network.

My laptop is connected to the unit’s 2.4 GHz network, and still has internet access through the unit.