Deleting all photos in iCloud I want to delete all my photos from my iCloud (ca. 2500 Photos), so that I have no photos in the iCloud.

I have gone into my iPhone's Photo app and selected all 2500 Photos and deleted them. Then I deleted them from the "recently deleted" album in the Photos app. It said that this would delete them from all devices connected to my iCloud. 

Now, my iPhone's Photos app is absolutely empty, which is what I wanted.

However, 24 hours later, on my other iOS devices, as well as on iCloud.com (accessed through my laptop), the Photos are still visible, even though it says "Updated".

Does it just take some time for the deletion of all photos to sync to all iOS devices/iCloud.com, or is something not working as I expect it to work?
It could take several hours to delete photos from iCloud. You're deleting 2500 photos so it could take more than 24 hours to delete them depending on when you've plugged your iPhone into power and connected it to wifi. 

You are aware that once these photos are deleted they're gone for good (unless you have another backup somewhere) and you can't get them back?