Nothing special on the router side. The server machine is connected normally, the same as other devices in my house. I have written a blog about how to enable relay mode for ipv6 for the secondary router, you can check it out here blog.bimal1412.com.np/enable-ipv6-on-a-secondary-…
As, for the other question, yes I am sure it will get tedious if you plan on using it to host something with ipv6 directly from the docker container without tunneling. But I am sure that can be solved by writing a bash script to automate the config to get the new ipv6 gateway if the IPv6 prefix changes.