This was interesting! It’s always neat to see more Nix/Guix posts here—the stealthy future of packaging systems, 
.
As one small note…
does not fit Fly.io’s model of one process per machine (VM)
This was true for a long time, but they relaxed it recently—although the new way† isn’t necessarily better in this case.
†Although possibly you’ve seen this already, since there’s the phrase “default model” elsewhere.