IPv6 ULA with docker and fritzbox

I am currently trying to setup a pihole for my network using docker and IPv6 and I am facing some problems I could not yet find a solution to. In order to avoid additional complexity I am currently trying to get the network stuff right, before trying the installation of pi hole.

I managed to get docker on the raspi running without to much hassle using Prefix-Delegation to get a /62 Network assigned to my raspi. I instructed docker to use a /64 of this prefix for its default network (maybe later I will try using additional user defined networks to).
Using this I manged to get a basic docker container running which is reachable within the network by the global IPv6 (2001:xxxx....).
As I want to avoid the chicke / egg problem I now want to use ULA (unique local address) within the network. Therefore I setup my router (Fritzbox) to always provide ULAs no matter if there is an existing Internet-Connection or not.

This works quite well, all devices get a IPv6 address from fd00:xxxx:xxxx:0000::/64 as configured.
As I did not find a possibility to configure prefix delegation for ULA (I doubt this would be useful) there is no chance to use this mechanism to delegate a part of the prefix to use in docker.

So for now I tried to set up a a different subnet for docker like fd00:xxxx:xxxx:0001::/64 Running containers will get an address from this range and I am able to ping the address from the raspi and reach the docker interface on the raspi from within a container.

However I am currently struggling to get Routing within the fritzbox set up right. I entered the prefix fd00:xxxx:xxxx:0001/64 as a static route and put in the link-local address of the raspi (fe80:yyyy:yyyy:....)
However I am not able to establish a connection either from within a docker container towards a machine in the local network or the other way round.

Has someone got something like this working or am I just missing a crucial step/setting?

This has no ties to Pi-hole or DNS.

I'll just note that using public IPv6 addresses used to partially compromise container isolation for a long time, potentially exposing all ports regardless of configuration. That specific issue is closed now, but I am unaware if and how that has been addressed.

If you choose or have to stick with IPv6, you may improve your chances for an answer by also consulting sources on Docker or networking.