MacOS – How to automatically disable WiFi when I connect to an Ethernet network

You want to make sure that your Mac's ethernet device is before the wifi device in the system's network service order. In System Preferences, Go to Network; under the device listing, click the 'gear' icon and select 'Set Service Order.' Drag 'Ethernet' to the top. Per my understanding, all traffic will attempt to be routed through the top-most device, and will only be routed through devices lower in the chain should the above route(s) be unavailable.

The diagram at the top of the article you linked to in the question seems to show exactly what you are trying to achieve; the extended base stations are connected via Ethernet, from their WAN ports, to a network port on the primary base station. Have you tried this configuration?