I managed to get the project up and running using the sample shell.nix config, it is great how easy that was and how it worked out of the box. It will take some time to setup the language server (as I want to use elixir-ls), but having the initial configuration that works is the core thing I was searching for hours and couldn’t find.
For now I will stick to nix-shell, as it works and I actually understand at least partly what is going on there 
, but this doesn’t mean that @joelpaulkoch 's answer is not valid too, maybe I will try it later when I will have more time.
To keep maximum visibility for other people searching for this too I will leave both answers in this post and mark this post as the correct answer:
Thanks again @cmkarlsson and @joelpaulkoch for the swift response and detailed answers!
