Thank you for your reply to my query. Will removing the photos from my MackBook Pro also remove them from my iPad & iPhone? I’m currently connected to the “cloud”.
First make sure that all your photos from your phone are synced to your Mac.
Ok first once you have reconnected your Mac to your photo Library - you need to make sure it is designated as your system photo library.
Follow these steps to designate a System Photo Library:
Quit Photos.
Hold down the Option key and open Photos. One of the photo libraries has already been designated as YourLibraryName (System Photo Library).
Choose the library you want to designate as the System Photo Library.
After Photos has opened the library, choose Photos > Preferences from the menu bar.
Click the General tab.
Click the Use as System Photo Library button.
Then click the iCloud tab and enable iCloud Photos. You can choose to download original files and optimise Mac storage.
iCloud will then Sync everything back to your phone, so don't panic if your photos disappear for a moment, they will come back.
This does work as I did the same thing myself a couple of months ago. I moved by entire 500GB photo library to an external SSD.