Well, if you're generating JSON with Nix, you don't have to put everything inside of one file. It would be a better idea to split it up into multiple. You can also use builtins.readFile for reading config files which don't have to be generated in a complex manner. It's up to you to choose, I just kept everything inside of one file since it makes for a simpler example.

Edit: I have updated the documentation to mention this explicitly, thanks!