As far as I have understood, “Mangle” is the more advanced/complex way of doing PBR. Routing-Rules are the easy, fast way to do things - but with a lot less options.
But I think it is by design difficult to PBR WireGuard.
Is it not possible to say WG0 hat the IP 172.16.0.1, WG1 has 172.16.1.1 (all done via IP>Addresses).
If this doesnt work, the double-NAT-Workaround maybe helps?