I deleted 8000 photos from my iPhone however I still get the message “storage full”.

It doesn't automatically delete them. I believe it's 30 days where it's scheduled for deletion unless you manually delete them. They go to a "Recently Deleted" folder that you can find by scrolling to the bottom when you select Photos. It's kind of a pain but you need to select them and then tap on "Delete All".

https://support.apple.com/en-us/HT205856