Thanks for the suggestion. It turns out, I was wrong. dhcpcd is being imported into base for FreeBSD 13.
One big limitation with dual-dhclient is that dhclient doesn’t support setting the delegated prefix on the interface. You have to parse it out and generate a rtadvd.conf with the prefix.
I ended up switching from dhcp6 to dhcpcd since the latter is actually maintained and seems to work better with my ISP (hopefully …).
Colin Percival made it a bit easier (IMO)[1] it is meant for AWS EC2 but worked for me with my ISP as well. The guide you submitted probably will work everywhere.
[1] http://www.daemonology.net/blog/2017-01-26-IPv6-on-FreeBSD-E...