Multi-subnet mesh protocol?

I've got a few different networks I run on my main router. I would like to deploy a mesh network topology that allows all the networks on the main router (each with their own dhcp server and subnet) to be connected via a single ethernet cable to another router to expand wifi. What would be the best way to do this? VLAN tagging? Or do some mesh protocols support this feature natively?

Definitely vlan tagging of the additional subnets/networks.

A BATMAN mesh supports VLAN tagging directly. When the mesh is running there will be an interface typically named bat0, then you can use VLANs on it with the notation bat0.2 etc in config files just like an Ethernet port.