Assuming the phone is a newer model (not lightning cable), get a
USB-C external drive
https://www.amazon.ca/SanDisk-Ultra-...s%2C211&sr=8-4
These have been a revelation for me.
It plugs directly into the phone. No app needed, just mount for file transfer. Then you can simply just go into your file manager (whatever the iPhone equivalent of that is–I cant remember), select the photos, then there should be a move command. Select the USB, and 'move here'. Just like on the computer.
Or you can copy and paste the files to the drive and then delete the originals.
Put the drive on a keychain so it isnt lost since these ones are quite small.