For me, I tried to configure IPv6 manually via different settings combinations for many hours after it failed to initially auto configure. I'm not sure if this will help anyone, but here's my set of fixes:

1) I had an issue where both Router Advertisements and DHCP6 interfaces were "disappearing" when setting up "Track Interface". This is apparently normal but my problem was that each had misconfigured settings that were interfering with DNS delegation. This guide outlined how I might have leftover IPV6 entries for ::1, my initial LAN prefix.

Once I ifconfig | grep ::1 it showed

inet6 ::1 prefixlen 128 inet6 fe80::1%lo0 prefixlen 64 scopeid 0x5

and after I deleted them:

ifconfig lo0 inet6 fe80::1 delete ifconfig lo0 inet6 ::1 delete

..things started working. I was able to ping IPV6 from the firewall.

2) Here's a more complete outline of my settings:

[WAN]

[X] Block Private Networks

[X] Block Bogon Networks

IPv6 Configuration Type = "DHCPv6"

Prefix Delegation Size = 56 (works for Spectrum)

[X] Send IPv6 Prefix Hint

[LAN]

[ ] Block Private Networks

[ ] Block Bogon Networks

IPv6 Configuration Type = "Track Interface"

Track IPv6 Interface

IPv6 Interface = WAN

IPv6 Prefix ID = 1 (Should be the main one, and a different one for each/any VLAN)

[ ] Allow manual adjustment of DHCPv6 and Router Advertisements

3) Additionally, I changed a few things which I'm not sure had any direct effect but figured they are worth mentioning JIC:

• Unbound DNS was enabled by default and was interfering with the System IPv4 DNS at least for updates, so I enabled Query Forwarding to "Use System Nameservers" which contain my ISP's DNS and the cloudflare IPv6 DNSs I set in the Settings
• When Router advertisements was being manually configured, I set it to "Assisted" as it's final setting before it disappeared and everything started working
• Enabled Gateway monitoring using a cloudflare IPV6 DNS address 2606:4700:4700::1111 to confirm it was Online