I assume you have the WG server enabled on your Flint 3, and you have also enabled the VPN client and connected it to NordVPN with a dedicated IP.
In this case, go to Flint3's NordVPN tunnel -> 'From-Specified Connection Types', and check 'WireGuard Server'.
Once applied, your Slate AX traffic will be routed: Slate AX WG client -> Flint 3 WG server -> Flint 3 NordVPN.
Thank you Bruce, thats exactly what I wanted to achieve.
Edit
I can get this to work at both ends now by tagging the Wireguard and local network tabs however when I implement a client exclusion it turns of the function.
Added the NoVPN solution to exclude specific devices