Deleting Photos form iPhone but not iCloud

There is no save way to do that and still use iCloud.

But you could reduce the size of the photos on your iPhone.

Manage your photo and video storage - Apple Support

Store your photos and videos in iCloud

Your photos and videos are stored on your device in their original, high-resolution version. This means that they use a lot of space on your iPhone, iPad, and iPod touch. With iCloud Photos, you can make more space available on your device and have your entire collection, everywhere you go.

Get started with iCloud Photos:

1. Tap Settings > [your name] > iCloud > Photos.
2. Turn on iCloud Photos.
3. Select Optimize iPhone Storage to save space on your device.

With Optimize Storage, smaller, space-saving photos and videos are kept on your device while all of your original, full-resolution versions are stored in iCloud. And as long as you have enough space in iCloud, you can store as many photos and videos as you want.

Ralf

iCloud Photos is primarily a syncing service and not an independent external storage.

we should use iCloud Photos, if we want to keep or Photos Library identical on all our Apple devices and all adjustments, metadata, new photos perfectly in sync across all our devices. with iCloud Photos we all always be seeing the same photos, wherever we we are viewing our iCloud Photos, on a Mac, an iPhone, an iPad.

To save some storage locally on your devices, follow Ralphs recommendation to turn on "Optimize Storage". Then the system will automatically remove local downloads of some of your photos, when you need more free storage. But keep them in iCloud. But do not delete photos on your own, unless it is a photo that you do not want to keep.

Optimize Storage is fully automatic, you cannot control which photos will get optimized.