I actually fixed the issue, I'm now rooted and have TWRP decrypted.
In the bootloop which I got from flashing the magisk_patched boot image, I started a cmd on my pc and did the following commands via ADB:
-adb devices (to check if its online)
-adb shell
-magisk --remove-modules
This deleted all the modules, which caused the bootloop, then the phone restarted itself and the phone booted in a few seconds with root.
After that I installed the unofficial twrp Q installer from https://xdaforums.com/showpost.php?p=80884663&postcount=3190
Flashed Magisk 20.1 and rebooted, everything working fine now. Finally.
@StanvdT--doo-oo-ood--you are magic!
Great job, and thanks for posting your solution.