For what reason do you assign WireGuard interfaces?
The only reason to assign Interfaces with OpenVPN or WireGuard are for using VPN providers like Mullvad. Usually no business needs to assign them in any way. We have OpenVPN for remote users and WireGuard for mobile. We also have customers with nearly 100 branches connected via OpenVPN, no assigning needed. Both can coexist without assigning for sure.
Step 5(a) - Assign an interface to WireGuard (recommended)HintThis step is not strictly necessary in any circumstances for a road warrior setup. However, it is useful to implement, for several reasons:First, it generates an alias for the tunnel subnet(s) that can be used in firewall rules. Otherwise you will need to define your own alias or at least manually specify the subnet(s)Second, it automatically adds an IPv4 outbound NAT rule, which will allow the tunnel to access IPv4 IPs outside of the local network (if that is desired), without needing to manually add a ruleFinally, it allows separation of the firewall rules of each WireGuard instance (each wgX device). Otherwise they all need to be configured on the default WireGuard group that OPNsense creates. This is more an organisational aesthetic, rather than an issue of substance