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.
It is just a Linux device. Other people will install NixOS on it anyway, and use specializations if the whole idea of swapping device roles in-and-out is viable. I don't really understand why would the team that already got a full plate decide to also invent a whole new Linux system while they're creating their hardware device.