I don’t know if you’ve already considered it but you can very well add or replace a single module in your “stable” nixos configuration. Steps here:
[Question] How to enable a service from a different channel · Issue #41212 · NixOS/nixpkgs · GitHub
It’s a slightly complex module because of two services, but you might want to peruse #77734 PR on github to see how easy a module is (it’s just another nix file where you define the interface’s options by just defining some nix attributes)
Its the cheapest way in terms of effort. @bjornfor’s approach is probably better in most other ways.