PI-hole and IPv6 DNS server

PI-hole didn't work well as my ISPs IPv6 DNS servers allow for ads to go through. The router is provided by my ISP and it's not possible to disable the IPv6 DNS servers. However I can choose which DNS servers to use so I simply wrote some fake IPv6 addresses and it works. But I wonder, can this cause any problems?

Providing fake, non-existing IPv6 addresses may slow down DNS resolution for IPv6 capable clients, as they may unsuccessfully try those IPv6 addresses before falling back to IPv4.
If a client would do so for every single request, that may be quite noticeable.

Did you try putting in one of your Pi-hole host machines' IPv6 ULA or LLA?

If I enter ifconfig i'll get a fe80::xxx address. Only five groupings. Should I just use the fe80::xxx address? I read somewhere that the fifth (last) grouping may change at reboot so what I did was to write fe80:e3f0:3dee:9dce:0000:0000:0000:0000 so the last four groupings are not legit. My phone says IPv6 DNS server address is fe80:e3f0:3dee:9dce:: (double colon at the end).

ip4 dens server

For IPV6 :: is a substitute for a string a zeros ( only one set is allowed however). In your case it compresses 0000:0000:0000:0000 to ::
This is why your ipv6 lo address looks like ::1