Hello,
Just to let you know that my IPv6 is working now, again it seemed to be a faulty firewall rule. Once the connection established between my "pfsense" <-> "default isp gw" everything else went fine.
. public addresses 2800:160:17C5::/48
. internal addresses 2800:160:17C5:1~4::/64
Initially I messed up with dhchpv6, then manual ipv6, finally stateless :-) and it works all right!
Again, thank you all for your great comments and support!