The attached file in #13 above worked, thank you!
In case someone would like to now "How to do it" until next time this is how I did it:
How to install OOS update and root via created patch.img file in the Magisk app
1. Download the full version eg the OOS 11.x.x.x.zip file on the computer, and copy the file to the phone's internal memory as well.
2. Run the payload_dump program (
https://androidfilehost.com/?fid=1899786940962605222) to extract a boot.img from the computer. You have to extract the payload.bin file from the OOS 11.x.x.x.zip file and place it in the Input folder, then run the exe-file and wait for payload to complete, then copy over the boot.img file found in the Output folder to the phone.
3. Go to Magisk app on the phone, click on the top install "Magisk", click "Select and patch a file", search for the boot.img file you created.
4. Now, Magisk will create a patched magic.img file from the boot.img file (can be called eg magic_patched-23000_xt8QR.img)
5. Copy the magisk_patched-xxxxxxx.img to your computer's "Root folder" where you have stored ADB files, etc
6. Now install the OOS 11.x.x.x zip file (should be in the mobile's Internal memory) via Settings> System> System updates and the "gear"> Local upgrade
7. When done, DO NOT reboot but go directly to Fastboot mode + connect cable to the computer
8. On the computer, type from your "Root folder":
fastboot flash boot magisk_patched-xxxxxxx.img
9. Reboot the phone - you are rooted!