free space on iphone

I am paying for 2T of icloud storage. Want to keep photos and videos there but free space on my 256GB iphone. Do I understand correctly that I could: turn off Photos synching on the phone, delete them, but once I turn that synch back, it will delete them on ICLOUD which will be a disaster. Any way around it? Besides copying using an external? Then what is the point of paying for 2T of storage if I can never fill it up beyond the 256GB in the iphone?