> What's really nice is using direnv to cd into a directory and having a flake.nix file in it which changes the entire system to what I need to do in that folder.

How cross-platform is direnv + Nix? Do flake.nix files have macOS/Linux versions?

One of the biggest advantages of using Docker images as a package format is that it's (sort of) cross-platform via Docker desktop and such on a Mac. I would prefer to use something like Nix if possible if it solves the problem a different way (being aware of platforms).