Thanks, @cupora for reminding my question which I forget to answer my question even after two years.
First of all the scenario or the use-case elaborated above is conceptionally wrong because according to the OpwnWrt documentation, my OpenWrt is acting as a router but not as a gateway.
Definition: If a device facing a modem or has an integrated modem then it is the gateway on the other hand the device that is facing a gateway is a router.
Source: https://openwrt.org/docs/guide-user/network/switch_router_gateway_and_nat#openwrt_roles
The DS-Lite setup in the OpenWrt makes sense only if the OpenWrt acting as a gateway but not as a router.
Supplying the dual-stack (IPv4 and IPv6) down to the downstream routers is the sole duty of the gateway or the upstream router, in this example, the FritzBox which acts as a modem gateway.
OpenWrt has to be coupled to the FritzBox via the wan4 and wan6 interfaces and delegate the dual-stack further to the downstream routers or the clients.
As long as the FritzBox is getting the /56 public IPv6 prefix (Most of the German ISPs) it can delegate the /57 prefix to the OpenWrt in general. In contrast to the IPv6, the FritzBox is getting an IPv4 from the 100.64.0.0/10 CGNAT subnet because of the DS-Lite. Therefore FritzBox has to use the NAT to delegate the IPv4 addresses further down with the 192.168.178.0/24 subnet.
The wan6 interface will get one public IPv6 address and the PD and the wan4 interface will get one IPv4 from the FritzBox 192.168.178.0/24 subnet.
Additionally, just to avoid the double NAT the OpenWrt has to be the exposed host to the FritzBox.