Just buy a new SSD to install Linux on. If you decide to switch back just plug the old one in.

I quite enjoy Nix flakes for this. Only certain languages have good support though (C, Rust, Haskell, OCaml, …).

A package is reproducible if you use the same inputs, run the build, and get the same outputs.

The issue is that the build can produce different outputs given the same inputs. So you need to modify the build or patch the outputs. This is something that is being worked on by most distributions: https://reproducible-builds.org/who/projects/

NixOS is not special in that regard nor are all NixOS packages reproducible.

Nope, nix doesn’t ensure or require that the builds are deterministic. It’s not any better in that regard than other package managers.

It’s not really fully reproducible either.

The implementations mostly don’t matter. The only thing that you need to get right are the interfaces.

Well, most people installing Arch for the first time have no idea what a typical Linux install does under the hood. That makes it a worthwhile learning experience. The same commands you use during the setup you can later use to fix or change things. It basically forces you to become a somewhat proficient Linux user.

Some people consider working on programming languages fun, so they create new ones.