I have this running and it works pretty well. Remember you still only have one "Default" gateway - e.g ISP-1.
We statically route some destinations out of ISP-2. We also use PBR to route some stuff via ISP-2.
ISP Redundancy relies on the ability (or not) to ping upstream IP's to tell if the line is up and healthy or not, and therefore whether to fail over or not. So remember, if both ISP circuits are from the same telco and that telco have an issue, it could affect the ability for both of the firewall's ISP lines to determine which is healthiest. I experienced this rather shaky meltdown recently and basically had little option but to wait for the telco to fix their issue. So for best resilience you want to use different telco's and ensure their cables in the ground don't run up the same street to your building... You know, the street that has a JCB about to start digging the road up, and both of your ISP lines with it 🙄