The ISP route is ONLY used for the initial handshake. After that traffic is sent through the tunnel which is dependent upon the wireguard address structure additional routes if necessary and applicable firewall rules. So access to your LAN from external wireguard users or another wireguard routers subnet is perfectly feasible as well as local LAN traffic heading out the tunnel to another remote LAN etc................

You are spouting gibberish. If you want to have a serious discussion
a. provide a diagram
b. explain the wans at both ends ( static, dynamic, public or private)
c. provide configs of MT devices and remote wireguard device setttings
(minus serial number, any public wanip information, keys etc.)