That depends on what you mean by ‘safe’
I assume you mean by not deleting a photo that also synchronizes the deletion in the cloud as well
If you just want to minimise local file sizes whilst you still have plenty of cloud space, you can ‘optimise’ photos, with which the full quality copy will be uploaded to iCloud with a smaller version kept on your phone
If you don’t need direct access to photos that you want to preserve then you can use alternative cloud and/or drives. Make sure that you copy rather than synchronize and then you can delete the phone copy
The risk of drive storage is if the drive fails, and the risk of cloud storage is if using free storage that they may shut down in the future
Keep your important photos and files in multiple places
You can also change your file sizes, eg with photos & videos choose ‘settings, camera’ and adjust the quality settings
Use ‘settings, general, iPhone storage’ to see how much each app is using. You may manage to clear up some space by removing and reinstalling an app, or it can direct you to where your main culprits are to then go to the apps and selectively remove files within them