SLAAC .... NPT ....
Never used these, as they are 'not needed' ( ? )
I followed the pfsense documentation as mentioned above, and was a happy IPv6 user for many years.
I advise you to validate the pfsense documentation. There is no SLAAC, even as it promises beautiful things. No NPT.
This boils down to : set up a DHCPv6 server on every LAN - with a pool, so you can static DHCP map, as the old DHCPv4 days, your devices.
I'm saying this with any in depth knowledge, but : as soon as I read NPT, there are issues .... so, it must be a complex thing.
And I tend to keep things "simple", especially my Ethernet networks and everything that is related to it.