Meanwhile I updated to 19.07.02 and adjusted my script as well as the ifup's happened far too often now.
(I will compare it to your's once I got time to spare.)
Powered by LuCI openwrt-19.07 branch (git-20.057.55219-13dd17f) / OpenWrt 19.07.2 r10947-65030d81f3
Your FB config sounds fine to me.
The OpenWRT configuration was quite a hassle - particulary because I don't understand what the Use builtin IPv6-management flag does -- at the WAN4 interface for example it doesn't make any sense at all (or am I wrong?)...
Since the upgrade to 19.xx (see above) they moved this flag to somewhere else. One weekend I played around with it and disabled it as often as possible - IIRC it didn't make any difference.
I cannot check my config right now unfortunately, but IIRC you disable DHCPv6-Service and NDP-Proxy and switch the Router Advertisement-Service to "server mode". This way all clients will derive their own address from the advertised prefix (via SLAAC) and use that.
My setup is even more complex as I use a Raspberry Pi with PiHole as DNS blackhole:
- It will serve IPv4 via DHCPv4 (i.e. DHCPv4 at OpenWRT disabled)
- OpenWRT: Set the IPv4 DNS to the pi-hole
- OpenWRT: Set the IPv6 DNS to the pi-hole
- OpenWRT: You need to enable the 'O' flag in RA so clients will get the DNS server via (stateless) DHCPv6 from the pi-hole (you have to enable IPv6 / SLAAC support in pi-hole for this)
- OpenWRT: But disable the 'M' flag in RA so clients will use SLAAC (see above)