@JasonTracy:
That said, I'm interested to hear more about how you're doing this!
What ? :D I will outline the principle for non-tracking setups.
Comcast, I am not with them. But they supply a /60, I understood from elsewhere.
Try [Interfaces: WAN] (Advanced(Send Options=ia-pd 0)) and (prefix delegation: checked)
If you get the /60 on the WAN, then you can know your prefixnumber as the first 64 bits.
Let's assume you get a prefixnumber like 2015:911:abcd:ff80(::1) on your WAN.
The last placeholder (0) in :ff80: is actually the supplied space, your 4-bits equals 15 LAN's possibly.
Now in webgui pfSense you can make a LAN-1 static as 2015:911:abcd:ff81::1/64 or a LAN-2 static as 2015:911:abcd:ff82::1/64. (The space available is :ff81: tru :ff8f: ).
A (PC) serverhost on LAN-1 (:ff81:) could get a number issued by you, (not by DHCPv6), say 2015:911:abcd:ff81::1001.
Or you could config a DHCPv6-Server/RA with a pool like [2015:911:abcd:ff81::1051 upto 2015:911:abcd:ff81::1100].
You make your WAN firewall rules on a wellknown server fixed IPv6 address.
So when ISP pulls/changes your 2015:911:abcd:ff8::/60, then your IPv6 LAN's and public server are securely off-line.