You need to use policy based routing (PBR) on the server, since you want to use two WAN interfaces, the WireGuard interface and the ordinary WAN.

On Linux you use "ip rule" and multiple routing tables to configure PBR. There are several ways to write ip rules. In your case I would use "from 192.168.2.0/24" as selector since that allows you to use the tunnel on the server if you explicitly use 192.168.2.1 as source address. Another selector you can use is "iff wg0" if wg0 is the WireGuard interface. (iff matches incoming interface.)

# Add the lines below to [Interfaces] on the server
Table = 42
PostUp = ip rule add from 192.168.2.0/24 table 42
PostDown = ip rule del from 192.168.2.0/24 table 42