Hi,
as a follow up:
I got everything fixed.
There are multiple things which should be set up, otherwise the monitor won’t work correctly - 7680x2680 240hz.
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Use DP 2.1 and not HDMI 2.1, even if windows supports HDMI 2.1 8k as well, it won’t work on Linux
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Change the DP Version on your monitor to 2.1, Default was 1.5 @ my monitor
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(3. I re - installed Garuda, after I changed everything)
Afterwards I changed the Kernel to the Zen4 Version. Maybe its required as well.
The patches from the answer above should be already implemented and are not needed.
Thanks for your help guys and I hope this solution will help other ppl with the same problem as well. 