Deleting photos from iPhone

iCloud Photos is a synchronization service. Using iCloud Photos ensures that the pictures on your your connected devices are exactly the same. If you add a picture to your iPhone, then that picture will be copied to iCloud and then to your iPad and Mac, etc. Deleting a picture on your Mac will delete it every place else. All the connected devices are kept in sync.

It is the connected Photos Libraries that are kept in sync. If you have pictures stored on your Mac that are not in the Library, but rather they are kept in folders in Documents, for instance, then they are not part of the synchronization. If you have pictures in the Mac's Photos Library, but iCloud is not turned on in Photos Settions>iCloud, then they won't be synced.

It wasn't clear to me exactly what you were doing--you said "even without unsyncing my iCloud," but but you didn't say syncing from what, for instance.

You could do an experiment and delete some pictures, so they go into Recently Deleted, and see what happens. You'd have 30 days to change your mind.

You can't do that as long as the iPhone and Mac are synced with iCloud Library. That's because it's a syncing feature and not off-device storage. Any change to one device, i.e. addition, edit or deletion whether intended or accidental will be mirrored on the other devices.