PI-Hole only blocking 2% of the ads says UI

The blocking rate is just a piece of informational data - by itself, it is not suitable to judge Pi-hole's operation, other than that Pi-hole is working.
It is also expected to fluctuate over time (sometimes wildly), depending on what domains your network tries to access, e.g. if your network would only ever visit discourse.pi-hole.net, blocking rate would be close to zero, and if you'd spend time mostly on Amazon instead, it would go up.

That said, your debug log suggests that your FritzBox router is advertising its own IPv6 as local DNS server:

-rw-r--r-- 1 root root 161 Feb  3 17:13 /etc/resolv.conf
   search fritz.box
   nameserver fd00:<redacted>8d
   nameserver 2a02:<redacted>8d

This allows your clients to by-pass Pi-hole.

You should configure your FritzBox to advertise your Pi-hole host machine's IPv6 as DNS server or to stop advertising its own, see e.g. Unresolved ipv6 adress in my top list - #4 by Bucking_Horn.