First of all, I'm very new in the OpenWrt and IPv6 worlds, so my terminology might be inappropriate.
How could I prevent the assignment of global IPv6 addresses to my LAN devices? I don't want to directly expose them to the world nor to have to configure a firewall, but I just want them to have local IPv6 addresses only instead (beginning with fd2a:4bdc:... as per the first picture). This happens if I disable the DHCPv6 client on the PPPoE, but in this way I'm losing also the IPv6 /128 assigned by my ISP, which I would need for further management/routing.
If I'm correct, what I should do is to disable the PPPoE-obtained prefix delegation to LAN, but I didn't figure out how to do it. I tried to add option pd '0' on wan6, which uses the dhcpv6 proto, but the effect was not the desired one as the prefix was still obtained from ISP and delegated to LAN.