I applaud the effort, but this is not the best way to save the few hours it takes me to get a new machine up and running that I have to take every year or so.
What happens if your build script breaks? You have to work around it then troubleshoot the problem. Already there goes your time savings.
I keep my dotfiles in a repo, that seems to be at the appropriate level of abstraction. Easy to set up, easy to add to, I can understand everything its doing, if it breaks I can localize the problem and fix it exactly where it's happening.
Trying to expand the scope to include arbitrary package management? I don't think the tooling is there yet, that will make this sort of thing very painful. The dotfile convention has been brewing for decades.
Trying this will inevitably force you to make compromises, and compromises defeat the whole point. The work it does is fundamentally equivalent to a package manager, and existing package managers haven't managed to get it done yet.
If every Mac developer can be convinced to use a package manager, and every package manager can be scripted, (looking at you, Mac App Store) and every app with a modular architecture also either plugs into an existing system or implements its own package manager, then this becomes viable. But you still need a modular and easy-to-manage way to manage app configuration too. If you can get them all to use dotfiles, awesome. But they don't now.
Right now, invariably you're reduced to a pile of hacky workarounds. Any automation will have been done at the wrong level and will break constantly.
The last time I needed to set one of these up, I started to write down a checklist of things to do. I gave up after I realized that I was just pulling the checklist from memory, it already exists and writing it down wasn't going to do much good.
