I'm a user of mwan3 and contribute to its development in a small way by mainly providing feedback with my multi WAN setup and maintaining the beast of it's documentation on the OpenWrt wiki (feedback and contributors welcome).
This setup ultimately requires the use of a NAT6 firewall script. NAT6 is currently broke with fw3 and LuCI, so this is an important helper script to workaround this current limitation.
The NAT6 configuration requirements are explained in more detail on the OpenWrt wiki.
This gist is aims to document my configuration for others.