Plus plan user here. We consider using the API method list_folder to pick up a large Dropbox folder (thousands of images 100-5000 kb each) to populate the Media Library for a WordPress site. Individual files would cause minimal download traffic, as another service picks up files from there and serves images with a CDN. Transfer from Dropbox to WordPress might happen with WP All Import → Import Images from URLs. For those with good Dropbox API experience – does this sound like something worth trying?