I'm not paying for gigabit but I do have Comcast and pfsense. I tried /64 and never got it to work. I had to switch to /60 and then everything started working just fine. Below are the settings I have on WAN and a guest network. You can use the guest network settings on your LAN, just make the prefix ID different for each interface.
You mentioned NAT6, I dont think pfsense supports that but I'm no expert and havent looked into it.
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