S3 has bucket versioning if you want to have multiple backups. The S3 sync command also does differential backups; if you for example try to run the script over and over it will only upload new/different files.
I agree this is not the absolute most optimized solution but it does work quite well for me and is easily extendible with other scripts and S3 CLI commands. Theoretically if Borgbackup or Duplicity are backing up to S3 they're using all the same commands as the S3 CLI/SDK.
Besides, shell scripting is fun!
They are not. Both Borg and Duplicity pack files into compressed, encrypted archives before uploading them to S3; "s3 sync" literally just uploads each file as an object with no additional processing.
Differential backup here means that if a file has changed you only send the change delta, not the whole file. This is what makes it possible tobrun that kind of things every hour if needed even on large folders.
If s3 supported that and with the already existing versioning you'd have a pretty kickass solution; that's basically what you can do with rsync and a zfs filesystem for example