Regarding the OP question, I did some reading of old posts and the short answer seems to be that you cannot stop the clients IPV6 queries but if your not using IPV6 then no harm no foul as the device can't make use of the information given. There seems to be some ways to hide that information from being displayed on the stats page if really wanted to. I would suggest going through this old thread for info.
What you observe is perfectly normal.
Disabling IPv6 won't stop client software from requesting type AAAA records. e.g. nslookup would commonly send both type A and AAAA queries for a given domain.
It's also not unexpected to see a client use its link-local IPv6 address (range fe80::/10), even if the router wouldn't advertise a public IPv6 prefix (range 2000::/3) because it has no public IPv6 connectivity.
My plan is to 1. figure out why my main machine cannot do IPv6 queries. Does not make any sense. The local inet6 adresses are there and the router is set up as well. Something is wrong but I don't know what. And then after that is clear to me I'd then 2. set up Pihole so that it works with IPv6 as well. I assume the steps here are the same as for IPv4, I just get a static IP for the Pihole and set that as my local DNSv6 server.
I appreciate that you're trying to help but I want to achieve the opposite. Currently my main machine does not resolve IPv6 queries but I want it to do so.
Well, that's where it gets icky. Your router may require you to enter a GUA (a 2xxx: address, a global one). That's annoying because the prefix of your GUA may be subject to change by your ISP (it's the IPv6 equivalent of a dynamic IP address). It may allow you to use a ULA (a unique local address, the rough equivalent of a private IP address), but IIRC it doesn't seem like your router is set up to let clients have one. And I honestly don't know if a link-local address even would work.
IMO the ideal is to have a ULA, which won't change, that can be used to reliably address your Pi in IPv6.
DHCPv6 can set a static IPv6, but it's still subject to your delegated prefix from your ISP, I think. And lastly, your router may do IPv6 with SLAAC (autoconfiguration), in which case DHCP isn't involved (it just announces the DNS servers and tells clients what prefix to base their address off of).
So... comes down to exactly what your router can do/accept. I bought a whole new router that could run OpenWRT just to make IPv6 work exactly how I wanted it to.
So I've basically solved my initial problem and now I'm having the same problem all over again.
I figured out what the issue with my main machine was. Like I suspected it was because of the LAN connection. Apparently you need to activate IPv6 for the ethernet connection. And for my system it was not activated. So it worked via wifi but not via LAN. Anyhow I fixed that and got the expected bad score on that test site because the IPv6 traffic bypassed my Pihole as expected.
Then I went and set my Pihole as my local DNSv6 server as well. I went back to the test site and now I'm getting the same score I initially got.
This means that I again do not know if Pihole is correctly blocking all the IPv6 queries or if the DNS setup via my Pihole just does not work and the IPv6 requests aren't going through at all.
I really wish there was a way to test this. Doing a "ping -6 google.com" works on the Windows machine via LAN which does suggest it is working but I'd like to verify it somehow.
EDIT: One more thing to add. I'm getting the 57% you talked about for both the setup I have now with a DNSv6 via Pihole as well as with the IPv6 turned off entirely. This suggests that Pihole always blocks all IPv6 ads on that test site.
The reason I got 70% initially was because of my browser configs. A vanilla Edge wihtout any settings gives the 57% as you expected.