The thing to try in your case would be to add a so called floating route with a higher metric, for example 200, and without a gateway address, only the outbound interface configured.
First, you would need to disable default route generation for your WAN interface (Use default gateway setting in Network -> Interfaces section, Advanced settings tab in WAN interface configuration window).
Then you would need to create a static route in the WebUI Network -> Routing -> Static routes.
Since FRR package used for dynamic routing reads Linux metric as a 4 byte binary, it takes first byte as Administrative Distance and remaining bytes as metric, which for kernel results in a rather large decimal number. The required metric value is 16777416, which translates to 0001 00000000 00000000 11001000. There, 0001 is administrative distance, for static routes equaling to 1, and 11001000 is the metric 200.
Now, the WebUI validates metric value only up to 255, so to enter 16777416, it would require you to interface with the CLI, and edit /etc/config/network configuration file.