I am trying to configure multi wan failover at my office using the cable internet and a 5g backup. I have almost everything functioning properly, except when I'm configuring the firewall rules.
Under LAN, if I change the IPV4 * rule and set the gateway to the wangroup, then my chromecast and site-site vpn stop working. Am I changing the wrong rule?
Have you set monitoring ips for the gateway
When using Unbound for DNS resolution you should also enable Default Gateway Switching via System->Settings->General, as local generated traffic will only use the current default gateway which will not change without this option.
https://docs.opnsense.org/manual/how-tos/multiwan.html