In your usecase, it probably doesn’t matter. I usually suggest DIY with more disks and XigmaNAS for ZFS and RaidZ2 or RaidZ3 depending on disk size and number. The cost is usually in the disks, and I tend to prefer smaller disks and more for cheap replacements when they die, cheaper initial purchase, and getting more spindles so I can use cheaper “everything” and still get decent performance. I usually wouldn’t consider a 2 bay NAS myself, mostly because I’d just do what you’re already doing and plug in a large single external disk. In the past (15 years ago now though) the single disks lasted quite a long time, though I did buy internal disks and used my own enclosures. Recent Amazon reviews imply that is the best plan even today in that model because the prebuild MyBooks etc at 12TB or whatever are supposedly horrifically unreliable, but maybe you’ve had better luck.