Apple - How to mass delete photos on iOS after backing them up manually (no iCloud)?

On macOS, you can use ImageCapture to select as many photos on the iPhone as you want and delete them. You can also import these photos to macOS with ImageCapture. Using Image capture is good since it cleans up thumbnails, database entries as well as the full resolution images.

Accessing the exposed DCIM folder isn’t the same as actually deleting the referenced photos via USB and software Apple makes.