If you haven’t read through them already:

• Nixpkgs documentation on packaging javascript things
• Nixpkgs wiki pages for Node.js and Language-specific package helpers
• Search for relevant issues and PRs: github.com/NixOS/nixpkgs/issues?q=cockpit

It looks like cockpit has been packaged and moduled. Maybe look at cockpit’s package and module definitions for inspiration for cockpit-machines? It looks like cockpit is a hybrid python+javascript project and has been packaged primarily as a python package, and cockpit-machines is also a hybrid python+javascript project but maybe the python aspect is less central, as I don’t see a top-level pyproject.toml, so you may not be able to exactly mimic the cockpit package?