Hi @bodleytunes,
Looks like you got it working without my help! That's great. You're now looking to intercept some traffic within docker so that you can use the router to intercept traffic. I assume you have other containers that you're looking to have access to the overlay, right?
Yes you'll need to figure out the tproxy stuff and you'll need to figure out how to declare a container as another container's network. If you're interested this video https://www.youtube.com/watch?v=odMy4F_iPdU or this discourse post Add new service on a separate "green" network in docker compose quickstart have similar information and yes they demonstrate using network_mode.
I think you're on the right track!