LocalStack is cool, but don't forget that you always have an alternative to build mock infrastructure in front of your AWS deps. Don't call S3 deps directly - implement a higher abstraction using only the APIs you actually call and back it with your local filesystem (for example).
It's not a _lot_ of work, and making your dev/prod differences more explicit can make debugging much easier.
It's not a _lot_ of work, and making your dev/prod differences more explicit can make debugging much easier.