Pi-hole offers a DHCP server as a last resort measure, when a router would not support to configure DNS at all.
Enabling Pi-hole's DHCP server would then cover IPv4, handing out its own IPv4 as DNS server to DHCP clients, as most client OSs default to request an IPv4 via DHCP.
That's not the case for IPv6:
Most OSs would autonomously create their IPv6 addresses from the network information as advertised by a router, rather than requesting an IPv6 address through a DHCPv6 server.
In other words: Your router will remain responsible for advertising your network details.
Enabling Pi-hole's IPv6 support just would add RDNSS router advertisements for its own IPv6 to your network.
About the only time this would be useful is if an IPv6-capable router would not support propagating any IPv6 DNS server at all, or only supports to disable propagation of its pre-configured IPv6 DNS servers.
But Fritzbox routers do support a wealth of DNS configuration options, including DHCPv6 and NDP/RA/RDNSS (though I'd personally prefer to not propagate any IPv6 DNS server addresses at all, see e.g. Unresolved ipv6 adress in my top list - #4 by Bucking_Horn).
Is there a specific reason why you want to use Pi-hole's DHCP server?