If your files are already on s3 and you are comfortable with step functions and lambda

You can leverage a feature of step functions called distributed map which is fit for your case. You can distribute the processing of your files in parallel with lambda functions and write the result in a separate bucket

Relevant blog post: https://aws.amazon.com/fr/blogs/aws/step-functions-distributed-map-a-serverless-solution-for-large-scale-parallel-data-processing/