Your router is doing NAT66 (getting an IPv6 address on its WAN interface, and configuring another IPv6 prefix range internally for your house) as well as NAT64 (getting v6 on WAN interface, doing v4 ...
I'm about to lab this out but I want to get some feedback as to whether what I want to do is possible before I put the time into it. This was brainstormed last week by a couple people smarter than me, ...