This solution may be for you if you don't have the Internet bandwidth or local storage to download all your photos right now, but you'd like to have a backup copy or archive copy just in case.
Google offers instructions for how to use Google Takeout to request Google to transfer the photos directly to the destination cloud storage service in ZIP files.
A nice feature of this solution is that Google offers to repeat this backup every three months.
Since these destinations are not photo services, they do not host the photos as individual photos nor do they capture photo metadata such as the date taken. Some of this metadata may still be within the photo files but in the archive you won't be able to do things like filter or view by date. Still, it's nice to know you have a backup copy.
The source code (markdown) for this article can be found on GitHub.