So, I finally succeeded in getting root rights back.
First I did a factory reset and then I used the FastbootEnhance tool to flash the extracted payload.bin in fastbootd mode.
Then I transferred the necessary files back to the smartphone,
installed Magisk,
patched the extracted boot.img,
copied the patched boot.img to the PC,
rebooted into the bootloader and used the patched boot.img to boot the smartphone.
And now I had the rights back and could do a direct install with Magisk.
Yeah!
kind regards