It's rather easy to do this, for photos transfer, a powerful software is recommand, name iMyfone Umatehttps://www.imyfone.com/iphone-space-saver/, it provids two options for iPhone photo transfer:
1. back up your original photos on your PC and compress them on your iPhone
2. back up your original photos on your PC dan mass delete them from your iPhone
So, just it can not only transfer your photos to your computer, but also delete / compress them at the same time. And the photos backed up can be tranfered to your iPhone via iTunes, if necessary.