I'm still relatively new to NixOS, having switched all my personal systems over to it this spring/summer. I don't do cloud dev atm, so haven't explored that use case yet.

But I believe NixOPs is the canonical way to do what you're describing in production/at scale:

https://github.com/NixOS/nixops

https://nixos.org/nixops/manual/

(more below)

If you want a more experienced answer, I suggest asking on the NixOS forum or subreddit, people are quick to answer in both places.

https://discourse.nixos.org/

https://www.reddit.com/r/NixOS/

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More useful NixOPs guides

https://www.thedroneely.com/posts/nixops-towards-the-final-f...

https://ops.functionalalgebra.com/nixops-by-example/

https://nixops.readthedocs.io/en/latest/