nix flake update
nix flake check --no-build
git commit -a
nh os switch
Is the routine I've settled into. Flake update because I use flakes, flake check because it's easier to see any warnings about deprecated options and the like so I can fix them preemptively, git commit after the check to avoid back-to back commits where the second is fixing some issue with the first, and nh because I like the pretty dependency graph and progress bar.
I honestly don't know how nh works under the hood, but it does seem to do concurrent builds, so it's probably something like that.