Give a small machine a clear job.
A kiosk, a lab machine, a local service box. Start with the job, describe the required software, and build an image you can recreate.
Build the image from the configuration.
Nix can build NixOS virtual machines and bootable installation images. That makes an image definition something you can review alongside the rest of your configuration, rather than a disk that only one person knows how to prepare.
Upstream references (2)
ARM is a platform, not a compatibility promise.
NixOS supports aarch64, but board support varies. Boot firmware, storage, graphics and the available kernel can determine whether a particular device is a sensible choice.
Check the documentation for the exact board. An ARM image is not a universal installer for every ARM computer.
Upstream references (1)
Updates need a recovery plan.
For an appliance, decide how an update is delivered, who approves it and what happens if connectivity disappears. Decide what data must survive and how the device is recovered when it cannot boot.
Those are design decisions for your deployment. This guide does not present NixOS as a turnkey fleet-management service.