Current FritzBox models should support deactivating IPv6 DNS servers via Home Network|Network|Network Settings|IP Addresses|IPv6 Settings.
If your 7530 is currently handling DHCP and DHCPv6, then you could try to:
a. Untick Also announce DNSv6 server via router advertisement (RFC 5006).
b. Tick Disable DHCPv6 server in the FRITZ!Box for the home network and
b.1. choose There are no other DHCPv6 servers for the home network.
This will have clients construct their IPv6 addresses via auto-configuration (SLAAC) exclusively, and will leave them with just an IPv4 address for DNS.
The one potential drawback would be that IPv6-only clients would have no means of DNS resolution. IPv4-only as well as dual-stack clients would still be fully satisfied with an IPv4 DNS server.