When your iOS device is using the iCloud photo storage option, you can delete all the photos using the web interface at https://icloud.com as opposed to using Image Capture.
You can "delete" all the local copies of the photos by navigating to:
- Settings > iCloud > Photos
Tap the iCloud Photo Library button and then tap "Remove from iPhone" which will delete all photos off the iOS device in question (if it's not an iPhone - the prompt should say iPad or iPod).
This failed me on my iPhone 11 Pro and iPad Pro with iOS 13 so they have residue photos left. In one case 36,000 photos that take up 45 GB of space. I will have to look at a second step since this does not seem to be as reliable at cleaning with recent hardware and software. Image Capture of course works, but I’ll see if I’m clever enough to do it untethered to a computer.