How to set up IPv6 for Pihole correctly?

Well, I have the same one (next to a 7530) and disagree. They are rated based on simplicity of configuration and hoe the average user can use them. That's all. I bought a WRT54GL some ten years ago and used it for a long time. It ran OpenWRT and gave me the full control of everything. And it was on the order of 50 € back then admittedly, it didn't have an embedded VDSL modem as this didn't exist back then). Just to put this in comparison... Yes, AVM uses Fritzbox to make a router that tries to fit everyone. However, they still limit heavily what can be configured by the user and what not. Giving less options also makes the support easier because there are less possibilities to break things.