A few thoughts, although probably not adding much… I have used Synologys in the past, still have one in the office, and actually have one arriving today (although I went with a much cheaper version this time, a DS417Play 4 bay unit, since it will be used exclusively as a destination for clone backups and for SynologyDrive, so I don’t need all that much power in this unit.
I agree that creating a “working” and a “backup” volume on the same set of hard drives is not really the way to go, since drive failures then affect both the working and backup drives. If I were using a very large drive-capacity unit (say 8 or 12 or 24 drives) I would create one volume on a subset of the drives and the second backup volume on another subset of the drives, but in general I think either a RAID5 with 1 or a RAID6 with 2 parity drives makes the most sense, with the backup on a completely separate volume.
Remember that RAID is NOT a backup. RAID protects from a single drive (or 2 drive in RAID 6) failure, but DOES not protect against data corruption or accidental deletion. For that, I strongly favor versioned backups.
I also feel strongly that the 3-2-1 principle makes sense for backups: 3 copies, on 2 different media, one offsite.
With a Synology, there are a few approaches to backups. Most, if not all, Synology units around today have USB ports and support connecting an external USB drive which can be used as a target for backups, using for instance Synology’s HyperBackup package. For you, with 15-20 TB of data, that isn’t really going to work, however; short of a hardware raid USB drive (I think WD has one that can go to 24TB using 2 x 12TB drives in RAID 0) you aren’t going to have sufficient capacity. Other hardware raid solutions are also very expensive, although I believe OWC is presently having a sale on some older hardware raid 4-bay units that support hardware raid.
A second option is a second Synology as a target for HyperBackup, but once again, you are talking some real dollars here.
One thing to consider is how much of the data you have truly needs backup, and is there a subset that you cannot afford to lose, and some that you would not care about? If the amount needing backup is small enough, an external USB drive might do the trick, especially with 12TB drives on sale this week at Best Buy! (You can, btw, attach several external USB drives and have Synology build them into a RAID volume, but I don’t advise this, as if one is disconnected accidentally the entire RAID array will fail.
Some people have bought Drobo units and connected them to Synology, and they do allow for creation of very large pools of data, but I have not been confident that the current Drobo models work well unless connected to Windows or Mac systems where there is a kernel-level extension that controls their operation. Others may know more about this that I do.
For offsite backup, unfortunately that is an expensive proposition using only the Synology, which natively supports Elephant Drive, HyperDrive to various targets (but, as far as I know, not BackBlaze B2), Glacier, and Synology’s own C2 backup system. The problem is that all of these are pay-as-you-use backend storage which means that with 20TB and growing, you are looking at again some significant spending.
@MacExpert suggested BackBlaze, and while BackBlaze does provide unlimited storage at a great price (and now allows you to pay extra to extent storage for deleted files and older versions to 1 year or permanent for an extra cost for each service), it does not provide backup for NAS units or for network mounted shares as part of the standard BackBlaze unlimited service.
One approach is to make a clone of the Synology data to a Mac and have BackBlaze then backup the clone, which will work, but in your situation that is not going to be feasible as you would need enough space (20TB or more) on the Mac for the clone.
Backing up such large amounts of data is a very difficult problem to achieve at low cost! I would be interested in hearing from anyone who has found a solution to this problem.