A colleague of mine developed this tool to make this functionality available in a CLI: https://github.com/chanzuckerberg/s3parcp
Just last week I wrote basically the same thing as an ad-hoc solution using boto3 because I had 10s of TB of data to pull out of Glacier and distribute across S3 buckets. It wasn't a big deal because I'm experienced writing parallel network code in Python and having big datastreams flow, and boto3 has good documentation, but things like this really shouldn't be left as an exercise to the SDK consumer.
I've been thinking of starting using Go to deploy some stuff that doesn't need python as dependency, and statically compiled :P
Edit: In this case for DO Spaces. Way more cheap.
RClone: https://rclone.org/ S3 Backend: https://rclone.org/s3/
For instance, if you "cp -a" a directory and then apply sync, it could do nothing and return success if the copied files were last modified before the ones in S3.
For our use case at work, we wanted to be _sure_ that sync always work as intended, and thus ended up recomputing etags locally and compare to the ones in S3 to know what to sync (got bitten by the issue of last modified before)