How to replace or delete entire iCloud photos library?

My iCloud Photos library is corrupt. Photos are missing, the structure is broken. I have a backup of the local photos library with all full res images. I'd like to replace the entire content on iCloud with my local data but am unable to.

My library has more than 40,000 photos and is 200GB large.

I tried:

• Create a new local, empty library -> it will download all from the cloud instead of replacing the cloud
• Delete all from iCloud.com -> returns a server error during deletion
• Delete in chunks of 1000 images -> "succeeds" but photos are not deleted
• Of course I turned off iCloud photos on all of my devices

All I'd really like to have is a fresh start...possible? If yes, how?

Since deleting directly in the browser did not work (I guess Apple never tested it with a library that large...), I ended up using this workaround: create a new Photos library (start Photos app with option key pressed). Make it the system library and sync with iCloud (only download previews). Then delete all photos and albums once the download has been completed. The changes will take about 15 minutes to be reflected in the browser. Then I switched back to my original library and enabled iCloud sync. Now it's uploading all of the >40,000 images back to iCloud.