Properly delete and then restore everything in iCloud Photos Due to what seems to be a very unusual bug, the storage counter for my iCloud account is stuck thinking that I have occupied more Photos storage than I really have. I was briefly in contact with Apple support, and I want to empty iCloud of photos and videos completely (basically down to 0 bytes). I want to do this to make sure that the storage usage is properly calculated.

After deleting all Photo/Video content in iCloud, I then want to restore the photos, videos, and preferably also all meta data (dates, folders, locations…) in its entirety.

Question: What would be the suggestible way to accomplish this?

I have Photos on my iMac set to "Download and keep all originals". Can I simply make a backup of Photos Library.photoslibrary, then go to iCloud.com and delete everything in the "Photos app", and somehow restore everything from the photoslibrary file?

I want to be absolutely sure that everything on iCloud is properly saved and restored. Even if that means that I have to download everything from iCloud.com – in case that the photoslibrary file for some discrepant reason may be incomplete relative to what's actually in iCloud.
"Download and keep all originals" is the correct setting to get all your photos onto your Mac. These originals are stored in Photos Library.photoslibrary, you can check the exact location of your library by launching the photos app while pressing option.
You can also check for syncing problems by creating a smart album, looking for files failed to sync.

After you have double-checked all of the above and saved your library on a secure place you go to System Preferences -> iCloud and choose Manage in the lower right corner (I don't know the exact translation, I'm using my Mac in another language). In the popup menu, choose photos and then delete from iCloud Drive.

The photos will be stored online for the next 30 days, so you are able to download them again, in case you made a mistake. After that period (make yourself a calendar entry) you can activate iCloud Photo Library again, and all your files will be uploaded again.