AAAA records have been introduced to DNS almost three decades ago. Pi-hole has supported them right from its start.
There is no performance gain in a local network - queries will be handled with the same speed, regardless whether they are transported via IPv4 and IPv6.
IPv6 may put its protocol advantages in effect for longer multi-hop routes, i.e. when a client in your network communicates with a public server on the Internet - when compared to an IPv4 packet taking the exact same route, that is!
Querying Pi-hole via IPv6 does not speed up that communication - it does not affect that communication at all. It's completely irrelevant how your client has determined the IP address of its peer.
In the current transition phase, it is difficult to judge whether you would overall benefit from IPv6, e.g. your favourite website may run IPv4 servers world-wide so data is served from the closest node, while it may run IPv6 only in its head-office country, so IPv6 packets may actually take longer. Also, there are still a lot of IPv4 only sites out there.
It's true that IPv6 offers certain advantages over IPv4, but organisational and computational benefits (e.g. larger address space, easier renumbering, faster routing, less load on routing nodes) lie mostly with ISPs and Internet infrastructure providers.
None of those are really relevant within a home network.
For a home user, the only tangible advantage I can think of would be if you would run multiple publically accessible servers on the same port from your home network, which is but a bit easier and cleaner with IPv6 than setting up multiple forwards for different ports with IPv4.
Pi-hole does not configure a Stateful DHCPv6 server.
Enabling that option will have Pi-hole advertise its own IPv6 as DNS server via NDP/RDNSS RAs and reply to a client's Stateless DHCPv6 requests by adding the following lines to your Pi-hole's
I don't know how that differs from your router as I don't know your router.
(I may not even know that if I would know your router.)
In general, you should configure your router to advertise Pi-hole's IPv6 address, or not offer any IPv6 DNS options at all.
So Pi-hole's option is primarily there to handle the case if your router wouldn't allow you to supply an IPv6 DNS server address, but only allow to switch off DNS options for IPv6.
Even then, I'd recommend not to use it, as IPv4 is fully sufficient to handle DNS in your home network.