Moving large numbers of files - like the 50k of Google Photos the author mentions - can be done using https://rclone.org. Set up Google Photos as a source and then use the copy command to move it to a different location[1]. There are a lot of different commands which work with many different cloud storage systems e.g. sync everything one way locally to a backup, mount a FUSE filesystem, do two-way sync, etc.
As others have said Google Photos backup via RClone has some caveats, mainly not downloading at original quality. Google Takeout does solve this but but in weird hierarchies.
I stumbled on a new project which simulates a browser download using a headless Chrome Developer tools session[0]. Looks interesting, supports continuation and can be ran on a cron job. Worth a look.