At one point at my previous employer here is how I tackled this. I monitored the Private IP on the serial connection for the carrier side of the circuit. This was done for the exact reason you mentioned checking for a layer 3 issue. I would monitor via ICMP only for each interface and labeled them accordingly. This ensured that I saw every outage even though just like in your scenario I had redundancies.
You can indicate that devices you need in the Orion Rport Writer.
Open the template and go to "Filter Results" tab and
click on "Add a new elementary condition"
on "Records where _*_ is equal -*- " select "IP Adres" and is equal xx.xxx.x.x(device ıp adres)
I hope it will help you