Odds are strongly against a 2 drive failure at your scale, though it does happen. I set my NAS up about 8 years ago, with 6 drives with raid-6 (well zfs’s version) and in that time two drives have failed years apart. When you get two hundreds of drives total in your operation you will see a dual drive failure.
Though you still really should have backups of everything you care about. Even though odds are in your favor someone reading this will lose data in their life on their NAS system.
At least you get updates. I’m running TruNAS core which isn’t updated anymore, and I have some jails doing things so I can’t migrate to scale easially.
The good news is this still works despite no updates it does everything it used to. There is almost zero reason to update any working NAS if it is behind a firewall.
The bad news is those jails are doing useful things and because I’m out of date I can’t update what is in them. Some of those services have new versions that add new features that I really really want.
I have ordered (should arrive tomorrow) a N100 which I’m going to manually migrate the useful services to one at a time. Once that is doing I’ll probably switch to XigmaNAS so I can stick with FreeBSD. (I’ve always preferred FreeBSD). That will leave my NAS as just file storage for a while, though depending on how I like XigmaNAS I might or might not run services on that.