Failover default rule change
I think the default rule should use an external IP, (we currently use 18.104.22.168) perhaps default to the DNS IP configured on the WAN interface for which the rule is written. Using the gateway IP is useless, as that will usually be hardcoded on the interface of the device. It will only be down if the device is off/disconnected. By using an external IP, you can actually see if the line is down. Using provider DNS may have other issues, but getting beyond the CPE is critical for monitoring.