The post isn't mentioning DHCPv6-PD[*] (prefix delegation.) I feel like murphy's law is kicking into effect right now and the ISP-provided router actually supports PD to give downstream routers their own /64.
(Or maybe not. Who knows. I feel like the post would've mentioned PD if they tried it.)
Also:
I divided it into a smaller subnet 2001:db8:abc:123:40::/76
Anything on a broadcast/multinode segment that isn't /64 is heresy ;)
(Or maybe not. Who knows. I feel like the post would've mentioned PD if they tried it.)
Also:
Anything on a broadcast/multinode segment that isn't /64 is heresy ;)---
[*]: https://en.wikipedia.org/wiki/Prefix_delegation
[*]: https://tools.ietf.org/html/rfc3633
[*]: https://github.com/openwrt/odhcp6c (-P option)