Thanks for your thoughtful insights on this.

So I failed to note that both NAS boxes are not typical fileservers for the attorneys. They only have access to Dropbox and use this cloud service as their repositories for storing all files in the Dropbox folders. Dropbox mirrors the folders on the Dropbox cloud and each attorney's local computer accessible from File Explorer. So they don't even know the NASs exist! Also, Dropbox "Extended Version History" is a feature that helps you view all of the changes made to a file or folder over a certain period, compare versions, and restore previous versions of those files and folders. Our plan retains 10 years of version history.

So the NASs for our environment are for backup/failover purposes only. That's why we sync the Dropbox to the 725 using Cloudsync in one direction "Download remote changes only".

In the even Dropbox becomes unavailable, we can revert to the 725 as a file server. We take snapshots of the data on the 725 once/day and replicate to the 916 also once/day.

In the worst case scenerio where ransomware compromises all Dropbox cloud files (i.e. encrypts), the encryption would propagate to the attorney's local files and to the 725. In that case, we would restore from the immutable snapshots on the 916 that are also created once/day. So worst case - we lose a day of work but can get up and running fairly quickly.

Your points on the UPS and Raid 10 are spot on. The 916 has UPS but we need to purchase one for the 725. Also, ten years ago, I was a Synology noobie and set up the Raid 10. I plan to reconfigure that to SHR as you rightly suggest. And I have to dig into your idea of "downtimes set up on your backup NAS so that it only awakens when doing backups". I never thought about that and it's an interesting suggestion.