Policy Based routing is the way to go.
Problem is that traffic which comes in via the WAN to the WG server also has to go out via the WAN, but to get internet access via the WG client the WG client has to route all traffic out via WG interface instead of the WAN.
If you want all traffic via the WG client then you can route all traffic via the WG client by default and only route the WG server port out via the WAN
If you want some LAN clients and/or some destinations going via the WG interface and some via the WAN then remove the default routing via the WG client by Disabling Route Allowed IPs and use PBR to route the desired LAN clients/destinations via the WG client, all described in the PBR guide