Best practices for backing up to Amazon s3 and a NAS

I would suggest you take a look at AcloudA that can be used as hardware gateway that presents cloud storage as local DAS to your server and offloads data to the cloud.

It can be installed either in local server or NAS (it depends on hardware specifics that you use) as SATA/SAS HDD with connectivity to cloud (either Amazon or Azure) allowing to run backup jobs as it would be done to local storage.

It really depends on your file size and number of object Storage GW from AWS costs you on monthly basis and you have caching option which is good.

CloudBerry Backup has block level backup where huge objects can be offloaded on incremental block level basis (basically changes goes to S3) what gives you another recovery point.

There is also real time backup that you may want to try.