I can't guide you much further.
I specifically cannot comment on your router's configuration, as I do not know it.
As far as router configuration is concerned, your router's documentation and support channels are probably a better source of information.
In addition, to better attract those of our community users who would perhaps use the same router as you do, consider including your router's make and model in your topic's title here.
For a start, note that local IPv6 DNS configuration differs from IPv4.
An IPv6 client may request DNS server information via DHCPv6 from your router, or it may pick it up from the advertisements your router broadcasts via NDP's RDNSS.
It would depend on the client which method it would use to figure its DNS configuration, e.g. Android will never use DHCPv6, and MS support for RDNSS only started with Win10. In a network with mixed clients, you have to make sure you meet their requirements and cover all of those options.
If you find your router to support configuration of a local IPv6 DNS server, make sure your Pi-hole is the sole such one.
As for the IPv6 address to use, you may list all IPv6 addresses of your Pi-hole's host machine by running:
ip -6 address show
You probably should avoid using a public GUA (range 2000::/3) for Pi-hole, as the IPv6 prefix is subject to be changed by your ISP (some ISP's switch prefixes on a regular basis, e.g. once every week or 24 hours) and the interface identifier may also change if IPv6 privacy extensions are active.
If your router supports it, you could consider to Use IPv6 ULA addresses for Pi-hole (range fd00::/8).
If it doesn't, using your Pi-hole's link-local address (range fe80::/10) is also an option, though that will only work if all your clients are on the same link/network segment as Pi-hole. For a single router home network, that's usually the case. Additional network equipment like switches and wifi access points may or may not split your network into multiple segments.
And as mentioned before, if you do not have to rely on IPv6 for some reason, you could also consider to disable IPv6 in your network as an ultimate measure, provided your router supports it.