iCloud

I'm assuming that you are using iCloud Photos ... correct? Is your goal to reduce the amount of photo storage used on your iPhone? One option, instead of what you are proposing, is to use the "Optimize Storage" option on your iPhone which will automatically manage the photo library on the phone.

Ref:

• Manage your photo and video storage - Apple Support
• Set up and use iCloud Photos - Apple Support

iCloud Photo is basically a photo syncing service. The second article I provided you covers the basics. The only method to remove photos on your phone without affecting what is on your iCloud drive is to first log out of iCloud on your phone, and then, delete those photos. The only issue will be once you log back in, the lower resolution versions will be synced back to your phone.

You're very welcome! I just wish I had a better solution for you. Maybe, someone else here, will chime in to offer you one. Good luck!