does it actually make sense to use four different custom dns servers? I thought, if one fails, then three others are available. Or is it better to choose two servers and get IPv4 and IPv6 from both?
You don't need any IPv6 DNS servers unless you run an IPv6-only network. An IPv4 DNS server can resolve all the DNS queries that an IPv6 server can resolve.
The way this works is that pihole-FTL chooses the best performer (which may not be predicated on a failure). It's generally a good idea to have more than one upstream server specified, in case one fails and that's the only one you are using.
And, even though we provide only four slots in the web GUI for custom upstream servers, you can manually edit file /etc/pihole/setupVars.conf and add as many as you like.
I had now installed Unbound directly from DietPi. There I adjusted my IP range in the Config again and entered the IP of Unbound (127.0.0.1#5335) into my PiHole Custom #1
This means that I no longer need to enter the DNS servers I mentioned above?
I continue to enter the block lists regularly via PiHole?
What should be displayed under dns-leak.com now? The IP of my RaspberryPi? Aktually its my Provider (Telekom).