Your FritzBox router propagates its own IPv6 address as DNS server.
Since the request for pi.hole is not answered with Pi-holes IP addresses, the FB obviously is neither using Pi-hole as upstream DNS server.
In that scenario, your client is by-passing Pi-hole completely, which would match your observation of less blocking.
You may want to refer to Fritz!Box (DE) - Pi-hole documentation for configuration recommendations.
To avoid IPv6 by-passes, you may also have a look at Unresolved ipv6 adress in my top list - #4 by Bucking_Horn.
If your other observation of maximum number of concurrent dns queries would still reoccur once you've settled for a clean router configuration, feel free to provide a fresh debug token.