uko gun

I ran into the same headache a few months ago when I had to copy box to dropbox https://mac.eltima.com/transfer-box-to-dropbox/ for a client migration. Downloading was not an option for me either. What actually worked was using a method described on mac.eltima — it lets you mount both Box and Dropbox like drives in Finder. After that, you can just drag the folders from one to the other without saving them locally. It saved me an insane amount of time and kept my SSD from filling up.