You probably already found it but regarding PBR see:
It finally dawns on me what it is you want.
But first pay attention to @trendy 's remark.
You are running a VPN server and client on the same router, which could be troublesome.
In order to connect to your WG server, traffic comes in via the WAN but should also go out via the WAN.
A VPN client might route everything out via the VPN so you cannot connect to your WG-server.
To make sure the WG-server always uses the WAN for its outgoing traffic you have to make in ip rule, see: No Wireguard handshake outside local network // Firewall help - #5 by egc
With that out of the way you are free to use whatever PBR and setup you want.
Traffic from your WG clients which connect from outside to your WG server have the source address of the WG client, that source address can be used to make a PBR routing rule.
As @trendy remarked it is possible this does not work (I will test it shortly but @trendy is the superior authority) and that you have to revert to making an ip rule just as described for your WG server above.
But we have not seen any configs so the above are just some general recommendation.
If it does not work then post configs:
Please copy the output of the following commands and post it here using the "Preformatted text </> " button.
Remember to redact passwords, MAC addresses and any public IP addresses you may have:
cat /etc/config/network
cat /etc/config/dhcp
cat /etc/config/firewall
cat /etc/config/pbr
ip ro
ip route show table all
ip ru
wg show