If the local VPN client was OpenVPN and running on the router, the answer would be fairly simple. If the default gateway was set to that of the local OpenVPN client, any internet bound traffic from clients of the OpenVPN server would automatically be routed over the local OpenVPN client. It's just automatic. If PBR (policy based routing) was active, you'd need to add the OpenVPN server's IP network (i.e., the tunnel) to PBR.
What makes your situation tricky is now YOU have to implement your own PBR to tell the router it should route the OpenVPN server's IP network over to the LAN ip of the device hosting the local VPN client (WG, OpenVPN, doesn't matter). And once there, it will be routed over that VPN client to the internet.
Certainly doable (I'm doing that right now on my own network, except the local VPN client is OpenVPN). But not an easy task for someone unfamiliar w/ networking. On the router, you'd need to create an alternate routing table, add the LAN ip of the device hosting the WG client as its default gateway, use ip rules to force OpenVPN server's IP packets to use that alternate routing table, etc. Again, doable, but it involves a lot more than just adding a simple static route.