Configure both as upstream. This causes OPNsense to use both for upstream according to priority. It will always use the GW online with higher priority, though there is no need for GW groups, but you can leave GW groups and firewall rules as they are.
You can leave gw group for that rule, the 'disable force gateway' option will override that, so don't care about it.Yes, lower number for WAN1 ia higher priority.First time I hear about a WG tunnel, but it should not be affected.