Maybe I’m misunderstanding what you’re looking for. Rclone has a Google Photos backend and an S3 backend. I haven’t used it but assumed you could simply copy from Google Photos into S3.

https://rclone.org/googlephotos/

"From March 31, 2025 rclone can only download photos it uploaded"

Also, it still requires to use my bandwidth or to have a VPS running and using it's own bandwidth