First of all the fdb4 address is ULA, which is private address, something like 192.168 in IPv4. So there is no point to redact in every post of yours the characters. This has local significance on OpenWrt, as the Sagem doesn't seem to delegate a ULA prefix, only the GUA.
Second the PC doesn't have all the necessary routes. It should have one for fdb4:6fc8:79c2::/48, not just the /64
Third, add option ip6addr 'fdb4:6fc8:79c2:b::1' in lan interface and don't let it decide which IP to use.
Fourth the static routes you have are pointless and invalid as all 3 addresses belong to the lan subnet.
Fifth add option ra_management '0' in dhcp/lan since you are using only SLAAC.
Sixth remove masquerade from lan if you don't need it. In general you are masquerading everywhere and I don't see the reason.
Also you have created a wan6 zone in firewall and you are masquerading there too.
There is quite a mess there, if it doesn't get fixed after all these I think it is a waste of time to troubleshoot any further. You'd better reset it to default, verify that it works (which I am sure it will even with defaults) and only add the vpns and the wifi related configurations.