OK, so IPv4 service should be of acceptable quality.
FritzBox web interface: Internet -> Online-Monitor -> IPv6-Präfix
The Archer C6 v2 comes with a QCA9563 SoC and should be able to route at this speed as long as you are not using SQM.
On the Archer C6, enable the DHCP server for the LAN interface and disable it for the WAN interface (which is connected to the guest VLAN). These are the OpenWrt defaults.
On the Fritzbox, disable the DHCP server for the LAN interface. Keep the DHCP server enabled for the guest network, which is the default. The guest network also serves as the WAN uplink of the Archer C6 and thereby as a transit network for the masqueraded LAN traffic.
The disadvantage is that any restrictions set up in the FritzBox now apply equally to guest and LAN traffic, but only for IPv4. I suggest to remove such restrictions where necessary and acceptable. Offering services to the outside world (port forwarding) now does not work anymore with IPv4, neither for the guest nor for the LAN network.
To deal with the firewall restrictions, you could route Internet traffic via the LAN interface of the FritzBox instead (swap cables between LAN1 and LAN4 ports, swap lan and guest SSIDs), and renumber your networks accordingly. Not recommended, because anybody looking at this network topology will become confused, perhaps including yourself in the future.
Two WLAN SSIDs are bridged by the Fritzbox onto their respective networks (LAN or guest). Packets to the Internet from wireless clients in the LAN segment pass the FritzBox twice:
LAN(wireless) -> Fritzbox -> LAN (wired) -> Archer C6 -> WAN=guest (wired) -> Fritzbox -> Cable
and I cannot guarantee the FritzBox will handle that well.
On the whole, I consider this solution rather untidy.
Why?
It would allow for a better solution:
Set up LAN and guest networks on the Archer C6 (both wired and wireless) with DHCP service.
Connect WAN of the Archer C6 to LAN on the Fritzbox. Disable WLAN on the Fritzbox.