We migrated the sites from 5.6.1 to 6.3 in the similar approach instead
of upgrade. I would recommend to use CRX2OAK. I have an article on my
blog about thisAdobe – CQ to AEM – Repository Migration – sree's WORLD
A shared S3 datastore can make all the binaries to sit in the S3 rather
than the instances. By setting the S3 configurations to make the
nodestore to keep minimal data, you can make almost all coming from S3.