For other people in this situation of running Nix the package manager on a non-Nix OS, a nascent Numtide project you may be interested in is system-manager:

https://discourse.nixos.org/t/github-numtide-system-manager-...

The idea is to provide some of the NixOS whole-system management niceties (especially controlling systemd units) but on non-NixOS systems like Ubuntu, in a way that behaves more like puppet/chef/ansible rather than competing with the native package manager.