Monitor config - 2 ISPs - Balancing only outbound traffic

Transparent certainly is the answer, the point is that transparent sends the monitor traffic with each pool member as "next hop". The number of router hops after that doesn't matter. As for the actual implementation there is no need to monitor each router with a unique monitor, you could but wouldn't you rather know that you can reach something through both routers? That's the very point of ISP load balancing after all.

So create a monitor and decide on something to be the end destination and put that as the Alias under Advanced properties and check Transparent. Assign that to the pool and you're done. And as I said, consider having multiple monitors and change the availability requirement.