There are quite some noteworthy alternatives like s3fs, rclone, goofys etc.
In terms of storing in s3 - is that in your buckets? Sound like the plan is to run the caching on your infrastructure, are there plans to allow customers to run those instances themselves?
Presumably the format within s3 is your own bespoke format? What does the migration strategy look like for people looking to move into or out of your infrastructure? They effectively pull everything down from their s3 to the local “filesystem”?