Have a read through below posting of mine:
If you have some IPv6 DNS settings on the router to play with, that rdisc6 tool can come in handy.
Yes you can use the ones that the Pi constructed from the IPv6 RA advertised prefixes (shown with rdisc) and SLAAC.
There are usually three types of addresses, GUA, ULA and LLA.
The boxes all the way at the bottom of below diagram:
Below one run on the Pi will show all currently configured IPv6 addresses (GUA, ULA and LLA) on all interfaces:
ip -br -6 address
Preferred one to be advertised as a DNS server is the ULA address (prefixes fc00, fd00 etc).
But if one is lacking or cant be supplied by the router, you can select the LLA one (fe00 etc).
LLA cant be routed though!
Better not advertise the GUA (2000::/3) for DNS to your clients bc the prefix for that one is under ISP control via deligation and can change and most often does change over time.
Bit similar as your public IPv4 address can change without you having any control over it.
If you have a Windows, MacOS or Linux client, you can test connectivity with below one in a command/terminal window:
nslookup pi.hole <RASPBERRYPI_IPV6_ADDRESS>
Also have a read through below FAQ with some router settings examples: