I finally called my ISP and asked them to disable the IPv6 DNS server in my router, understandably the super nice tech guy told me they can't do that, and that my problem should be the DHCP server, but that was already disabled... so we messed around the settings trying stuff with no success.
After some time he tried disabling "IPv6 support" in the DHCP settings, even though DHCP was already disabled, and lo and behold, that fixed it! Poor guy was baffled, no one ever asked him to do this before.
Now I get this when running rdisc6:
sudo rdisc6 eth0
Soliciting ff02::2 (ff02::2) on eth0...
Timed out.
Timed out.
Timed out.
And I can finally see my phones being filtered by Pi-hole.
Happy days!
For the record, the router is a Genexis Pure ED500 and the ISP is Adamo. If anyone stumbles upon this thread don't hesitate and call your ISP. 