Since IPv6 configuration is highly specific to your router's make, model and sometimes even firmware version, I can only provide some generic advice.
Note that DHCP is strictly an IPv4 protocol - the Stateful DHCPv6 flavour would be a close equivalent for IPv6 (but indeed a separate protocol of its own, using different ports). Also note that with IPv6, a client may support one or more of multiple ways to join a network (SLAAC/NDP, Stateless DHCPv6 and Stateful DHCPv6). It would depend on the client OS (and potentially its individual configuration) which ways it would support, e.g. Androids do not support DHCPv6 at all.
Your router's configuration UI may not make that clear a distinction when it comes to DHCP, though. You'd have to consult your router's documentation and support for exact details.
In addition to enabling or disabling DHCPv6 and the corresponding RAs for SLAAC/NDP, your router may or may not allow configuration of an IPv6 DNS server address. If it does, you'd have to make sure it would advertise solely Pi-hole's IPv6 address, preferably its ULA address (or in lieu of that, its link-local, as long as your network is one segment only).
Again, you'd have to consult your router's documentation and support channels for exact details.
Note however that some few router models show a misbehaviour of distributing their own IP address along with any custom address regardless.
In any case, as long as your router is advertising its own IPv6 address as DNS server, any device may by-pass Pi-hole via IPv6.
You'd absolutely have to find a way to configure your router to advertise your Pi-hole host machine's IPv6 as sole DNS server instead of its own.
You'd have to consult your router's documentation sources on further details for its IPv6 configuration options.
If your router doesn't support configuring IPv6 DNS, you could consider disabling IPv6 altogether.
If your router doesn't support that either, your IPv6 capable clients will bypass Pi-hole via IPv6.