dhcpcd-discuss

FWIW this is what I currently have, using OpenBSD and the ISP is Zen,
so it is very similar to what you're trying to do. In this case I am using
slaacd to configure SLAAC and default route, and dhcpcd only for prefix
delegation.

~~~~~ /etc/dhcpcd.conf
ipv6only
noipv6rs
duid
persistent
option rapid_commit
require dhcp_server_identifier
slaac private
nohook resolv.conf, lookup-hostname
allowinterfaces pppoe0 vlan2 vlan3666 vlan5 vlan3
script ""

interface pppoe0
  ia_pd 2 vlan2/1 vlan3666/2 vlan5/3 vlan3/4
~~~~~

~~~~~ /etc/hostname.pppoe0
mtu 1500
inet 0.0.0.0 255.255.255.255 0.0.0.1 pppoedev em1 authproto chap authname "zenXXXXXX@zen" authkey "XXXXX" up
inet6 eui64
inet6 autoconf -autoconfprivacy
!/sbin/route add default -ifp pppoe0 0.0.0.1
!/sbin/route add -inet6 default -ifp pppoe0 fe80::%pppoe0
~~~~~

~~~~~ /etc/rad.conf
default router yes
router lifetime 1800
interface vlan2
interface vlan5
interface vlan3
interface vlan3666
~~~~~