Not a very good answer.

If you have a Mac with sufficient storage, you can just enable iCloud to store photos locally, then export them. That could miss some data (I think descriptions you set for a photo were not exported the last time I checked), but it is most of the data.

I didn't answer because it's had and I'd rather not give a incorrect answer.

And, I may have conflated issues: Iphone Storage < Icloud storage vs opposite way.

Do you need to export them? Can you not copy the library file to a different drive?