I'd like to discourage you from manually setting a static IPv6 address.
IPv6 is heavily emphasizing auto-configuration.
You are introducing multiple dependencies to changes that you may not have direct control of, creating additional maintenance efforts to reflect those changes in your static IPv6, and you can easily cut your machine from IPv6 connectivity if you don't know what you are doing.
Besides, a network interface's link-local IPv6 address (starting with fe80::) already is stable by design, regardless whether its interface identifier is calculated according to RFC4291, RFC7217 or other similar methods.
I'd recommend to roll-back all of your related changes.
Then retrieve your Pi-hole host machine's true link-local IPv6 address, e.g. by running ip -6 address.
Take a note of that address and enter it as local DNS server in your FritzBox - with a German language model, that should be at Heimnetz|Netzwerk|Netzwerkeinstellungen|IPv6-Adressen: DNSv6-Server im Heimnetz.
Note that link-local addresses are only accessible by devices on the same link (commonly, those are all devices directly connected to your router)
If you wouldn't run a flat network, you'd have to use your Pi-hole host's IPv6 ULA instead (range fd00::/8). FritzBox routers can be configured to always hand out a specific, stable ULA prefix.
Avoid using a public GUA address (range 2000::/3): Depending on your country of residence and your actual ISP plan, that prefix would be subject to change regularly (like once a week or one every 24 hours) or upon router reboot or exchange.
If that happens, not only will your static IPv6 become unusable, but because of using a GUA, your DNS requests may even be leaving your private network (theoretically).
You should remove that entry.
By adding Pi-hole's IPv6 as an upstream Server of Pi-hole, you'd have closed a DNS loop.