Just some new docs on netmasks and making my wireguard setup less bad.
On the plus(?) side, I now have IPv6 routing to the internet, again, as the who knows how old ISP router here does not support IPv6. Previous encounters with IPv6 were when I setup lots of subnets with IPv6 (which went poorly, as some folks in the group that managed the network later claimed that IPv6 was not supported) or before that a hurricane electric tunnel.