Cool! More digging to do. I used ALARM to build some rpi images, it worked really well but alas we ended up going back to the base rpi os (Debian) due to the rolling release schedule making reproducible builds more difficult. I’m still thinking through the conceptual correct solution for an appliance…
As much as I am an avid arch linux fanboy, I don't think it's a good choice for "an appliance", the frequent update might cause unnecessary wear and tear for the storage chips if they use flash memory/sd card.
Lazy me would often update pacman database and upgrade just a few packages. Living dangerously but often times it doesn't really matter
If reproducible build is important maybe consider NixOS?