Hello HRoecker,
Thank you for using Apple Support Communities. Based on your post, you can't backup your iPhone XS Max due to the lack of iCloud Storage space.
For issues backing up to iCloud, try out: If you can't back up to iCloud
It sounds like your settings are reflecting a larger amount needed in iCloud Storage. We'd recommend to use the above to select only what you want to back up and deselect the apps you do not need to backup. That should allow you the space to complete the backup.
Also, in case it's helpful, see: What's the difference between device storage and iCloud storage?
Hoping this helps out, take care.