I'd recommend you check out rclone. It's a command line tool, so you'd need to be comfortable in the terminal.

https://rclone.org/

It comes with an adapter for Google Photos, so you can use it to download / backup photos and videos in your library.

Note: there are some limitations to this approach – namely that rclone won't be able to download your original image files, even if you uploaded with 'original' quality. These limitations are documented on the rclone website: https://rclone.org/googlephotos/