I don't have 'Upstream Gateway' checked, otherwise my config is the same as yours. Specifying a monitor IP is only necessary when the gateway doesn't respond to ping requests.
I think I found half the issue. As mentioned, I am migrating from pfSense. In pfSense, the values are specified in milliseconds whereas in OPNsense, they are in seconds. I have divided all my values by 1000 to get them into seconds and now my primary gateway reports stats as expected.My secondary gateway still does not show any stats however. Looking at my running services, I see dpinger specifically mentions my primary gateway, but not the secondary. Does OPNsense not support monitoring two gateways?