I think I understand your situation and the setup in the other thread should accomplish what you want, but in a slightly different way.
It only requires connecting the Mango LAN Port 2 to the main router in the same IP subnet and the default gateway would be the IP address of the main router. The Mango Wireguard client would connect over Port 2 through the main router to an external Wireguard server, even without the WAN port connected. In this way, the Mango would sit on the LAN as a “Wireguard appliance” and you can connect any client device on the same LAN subset with the default gateway being the IP address of the Mango to go through Wireguard. If the WAN Port 1 is configured as a LAN port, you can connect the IP phone to that port, although I have not personally used that feature.
EDIT:
In this setup, the Mango is in Router network mode, in order for Wireguard to work, and the router is not bridged.