Hi Volker,
Welcome to Microsoft Community.
I understand your frustration with the space requirements for the Windows 11 update on your Surface Pro 6. One workaround you can try is to free up space on your device by removing temporary files, uninstalling unused apps, or moving files to an external storage device. You can also use an external storage device to help you complete the update. To learn more, see Get more space with external storage.
Free up drive space in Windows - Microsoft Support
Unfortunately, Microsoft does not provide an option to split the update into chunks. I recommend freeing up space on your device to meet the update requirements. Let me know if you need further assistance. Your SincerelyHahn. W - MSFT | Microsoft Community Support Specialist