I am wondering how I can pin Wireguard to a gateway and/or cause it to fail over in a multi-WAN setup. This is only for outbound considerations as inbound - create the NAT and off I go.
After thinking about this for a few days and poking at docs, I don't think OPNSense offers an explicit way to do this. What I ended up doing that gets me most of the way there....
Wireguard will use the default gateway and has no concept of a gateway group. Therefore I simply set the priority of the WAN link I wanted to use for Wireguard to have priority and enabled "Allow default gateway switching."
Maybe this can help you ..
https://schnerring.net/blog/opnsense-baseline-guide-with-vpn-guest-and-vlan-support/