iPhone Storage
How can I easily remove thousands of photos from my phone but keep in iCloud photos to clear storage from my phone?
iPhone 8 Plus, iOS 13
You can’t. iCloud Photos is not off device storage. It is a syncing service.
Set up and use iCloud Photos - Apple Support
The photos and videos that you delete on one device are deleted everywhere that you use iCloud Photos. Photos and videos stay in the Recently Deleted folder for 30 days before they're deleted permanently.