That's how I started out with openSUSE + Nix, but I quickly found that I still had to learn about most of Nix's quirks, while only getting a small part of the benefits (namely, not the system configuration).
It may be worth it if you care a lot about having imperative escape hatches, but if you're willing to invest upfront work in getting past the learning curve and want to maximize the benefit you get from it, I would just recommend diving into NixOS directly.
I would say that it is a good gateway drug. I used Nix + home-manager on macOS for a while. It was really nice for replacing the broken macOS userland plus having reproducible dev environments.
I liked it so much that I tried NixOS on a Dell Workstation that I still had. Configuring a full machine declaratively was so powerful that I slowly moved from macOS to NixOS pretty much full-time.
It seems like it might be a nice way to get reproducible dev environments, without some of the headaches being discussed here.
Is this a common use, or do people typically just do all-or-nothing?