It's curious that you would choose NixOS for a system that "just works". As much as I like the core ideas of Nix(OS)—reproducibility, declarative configuration, snapshots and atomic upgrades/rollbacks—, having used it for a few years on several machines, I've found it to be opposite of that. It often requires manual intervention before an upgrade, since packages are frequently renamed and API changes are common. The Nix store caches a lot of data, which is good, but it also requires frequent garbage collection to recover space. The errors when something goes wrong are cryptic, and troubleshooting is an exercise in frustration. The documentation is some variation of confusing, sparse, outdated, or nonexistent. I'm sure that to a Nix veteran these might not be issues, but even after a few years of usage, I find it as hostile and impractical to use as on the first day. Using it for a server would be unthinkable for me.

For my personal NAS machine, I've used a Debian server with SnapRAID and mergerfs for nearly a decade now, using a combination of old and new HDDs. Debian is rock-solid, and I've gone through a couple of major version upgrades without issues. This setup is flexible, robust, easy/cheap to expand, and requires practically zero maintenance. I could automate the SnapRAID sync and "scrub", but I like doing it manually. Best of all, it's conceptually and technically simple to understand, and doesn't rely on black magic at the filesystem level. All my drives are encrypted with LUKS and use standard ext4. SnapRAID is great, since if one data drive fails, I don't lose access to the entire array. I've yet to experience a drive failure, though, so I haven't actually tested that in practice.

So I would recommend this approach if you want something simple, mostly maintenance-free, while remaining fully in control.