That doesn't sound like backup-- that sounds repeatedly damaging your fault tolerance and exposing yourself to risk while the production array rebuilds.
I couldn't consider this backup, either, because you're not creating multiple generations of copies and you're not storing those copies offline. (Backup means three things to me-- off-site, off-line, and multi-generational.)
I think you'd be better off using an actual backup tool. You don't mention your specific NAS model, but you could use any one of a number of tools to create backup sets that contained both full and differential backups that would allow you to recover multiple generations. (We don't recommend specific tools here-- there are certainly a number of products, both free/open source and proprietary in this space.)
Depending on the amount of data "churn" you may have enough bandwidth to perform the backups remotely.
Your NAS device may already have support for a protocol like rsync that would make this much easier, too.
If you're set on using hard disk drives as your backup "cartridges" you could easily end up with a situation where you have multiple disks that you rotate on / off the network to make multi-generational copies and to have an offline copy. (Offline is important. It's virtually impossible for a mistake or a malicious attack to destroy a copy of your data that's sitting in physically secure location disconnected from the network.)
Based on your comment you're just making copies of the production NAS onto a glorified external drive, then.
That seems reasonable from a "you're breaking your fault tolerance" perspective. I'd still want to have multiple generations and an offline backup.
I'd take that external drive back to the office and make generational backups against the office snapshot. Then I'd store the external drive offline.
Two weeks is a long recovery point objective, but if that's acceptable then so be it.