Advice/recommendations/pitfalls regarding implementing WAN failover?

[CJ]

I've finally managed to get my hands on a backup connection to help deal with the flakiness of my main connection.  Therefore I will be joining everyone in the fun adventure that is multiple WAN.

The documentation is straightforward but seems to assume that I only have WAN, WAN2, and LAN.  Unfortunately, my network is a lot more complicated with DMZ, IOT, etc interfaces.

Any advice or recommendations for someone entering into the world of gateway groups?  What pitfalls should I make sure to watch out for?  I'd like to avoid having to rebuild everything from scratch just yet.