So… this worked quiet well. I was able to deactivate SIP with the stick. Then I realised that my OS and Start Volume still contained data so I had to delete it and reinstall macOS. For some reason my Wi-Fi wasn't working anymore (no symbol on the top right). So I had to install macOS with a ethernet cable. After the installation I trief to continue the tutorial:
(GPU problem - Boot Hangs on Grey Screen… )
with Part 2: "Improve thermal and power management"
But the kext was not found so I assumed that with my reinstall it got lost.
So I reseted the NVRAM and went through the process again. It worked and I did everything until part "Preventive measures for future use". But why is my wireless still not working and not even appearing on my top right?
And is it right that when I boot all these "white texts of black screen" are appearing before the apple logo? Can/should I deactivate this? But a big thanks already!