I have a couple NixOS machines but I never put "just" and "NixOS" in the same sentence.

I strongly disagree here. On the technical side, I'm sure it works, I almost never hear about Nix not working.

On the practical side, "learn Nix" is a _massive_ onboarding task. Without Nix, I'd probably pick one up assuming I'll find something to do with it. With Nix, I'd wait until I have a project I know is worth figuring out Nix.

If this were my project, I'd probably go with the absolute most simple answer: multiple SD card readers. Install the base OS on one card, allow hot-swapping the other card, do some mount point stuff with the other card (like maybe it auto-mounts to /usr/local, and have packages install into /usr/local). Or maybe some kind of overlayfs with the other card. SD cards are cheap, and I'd rather glue an SD card holder to the back of a Flipper than learn Nix.

I had the same thought. Their stated goals are very much in line with what nix is all about.

Sadly, I totally get why they didn't. The nix way still isn't ready for "hey everybody, I'm making a thing, let's work on it together."

The hurdles are just too high for non-nix-nerds to pick it up while simultaneously trying to learn the underlying project.

It's a better way, currently only for those primarily in search of a better way.

I daily drive nixos and I have no idea what specializations are and reading that wiki article didn't help. Am I just looking at a way to drop in chunks of predefined config?

They already tried it and got so much pain that they decided on an easier way.