NixOS Flake setup that inherits from a Flake (Nix syntax issue)

while you use importApply to include every module separately.

That specific bit of code is not really relevant here. The flake-modules are a concept from flake.parts. Related to NixOS modules, but not exactly the same thing. flake.parts is a library that allows automating a lot of boilerplate that can be used cross flakes, but it is not necessary.

importApply is also a function from flake.parts.

Link in my previous discourse post also talks about flake.parts btw.

Maybe I don’t need to refer to the modules of the parent repository one-by-one, or at all? What would be the simplest way to make the inheritance work?

Depending on the purpose of child flake it might not be necessary at all. By sheer coincidence, there’s another post from today that talks about this.

You could also try using Haumea to auto-generate the imports.