I have coupled my personal nix configuration (packages, overlays) to my nixos machine-level config by setting
users.users.dan.packages appropriately, and I am not yet altogether certain I like it.
Upside: now I can run
sudo nix-channel --update && sudo nixos-rebuild switch on any of my machines and it takes care of all my personal preferences.
Downside: seems a bit unwieldy doing the whole git commit/push/rebuild just to add a package to my profile. But I guess
nix-env -i still works exactly as it ever did