So you have 56TB of total storage, but how much of that 56TB is actually used? Take the amount of storage used and add 10-12% to that figure. Now you create a new NAS (preferably off-site) with that amount of storage and that becomes your backup target. Take an initial backup (locally if possible to speed up the process) and then you can use something like rsync to create incremental backups going forward. This is the method I’ve used and so far it has worked out well. I target 10-12% more than the amount of used storage for my backup capacity because my storage use grows reasonably slowly. If your usage grows faster you might want to increase your “buffer” a little more so that you’re not having to constantly add drives to your backup target.
I mean, if you want a full mirror, rolling your own backup target is going to be the cheapest option even with the current high price of hardware. Other options are cloud storage, or using another media like tape. Cloud storage is of course an on going cost which rules it out for me, not to mention privacy concerns. There are certain “cold storage” options from cloud storage hosts which are considerably cheaper but they have limitations on how the data can be accessed and how often. The tape route is possible but it’s not really viable for home use due to the high upfront cost of the drives. Outside of that, backing up a subset of your storage as others have suggested is the only other option. Creating viable backups without breaking the bank is a challenge as old as computers, unfortunately.