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The company I work for open-sourced a straightforward library that does exactly that: https://github.com/embrace-io/s3-batch-object-store

(author here)

definitely!

I plan to add a batch write API. Also, an API where it buffers till it reaches certain size or a timeout to write to S3

This is why systems such as WarpStream regularly runs compaction jobs to more efficiently store objects and cut down on API calls.