The low-tech and not super resilient method is to buy a second 28TB NAS, put it in a different location and sync them periodically when you know your primary is in good shape.
Back in the days of DVDs, I used to backup my 20GB drive onto DVDs. I wonder if you could do something similar today but instead of a bunch of 4GB optical disks, you would use 4 x 8TB drives?
