Move data from iPhone to windows
I have had success with simply mounting and using File Explorer to locate the DCIM directory and copying it out.  However, I found it easiest to delete the photos from within the Photos app on the phone itself, despite how tedious that is.

You have to go into the Photos app, select the camera roll album, press 'Select' at the top right, and drag to select the photos you want to delete.  I recommend deleting in batches instead of trying to select them all in one go.  Unfortunately, this doesn't actually delete the photos, it just moves them to the 'Recently Deleted' folder.  Go to that folder, click 'Select' then 'Delete All'.

Yes it's tedious, but it works every time and I've gotten pretty fast at doing it.