Pushing WireGuard traffic out a specific GW using static routes requires a reboot to revert.
I initially had my WireGuard traffic going out a secondary WAN using a static route with the remote wireguard endpoint as a /32 route. This worked great. I'm now using a gateway group as the pfSense default gateway and so I no longer needed this static route. Simply removing the static route and clearing state table wasn't enough to get WireGuard honoring the change.