Linux

I'd like to argue the other way: jump into whatever you find interesting and do stupid shit.

Unless this is a computer that your job relies on or whatever, then it doesn't matter how much shit you break, as long as you're learning what to not do next time.

It's pretty much how everyone who is all 'oh no! be very careful and take small steps!' started, but they've just forgotten about having done that.

(I've broken so much shit both personally and professionally, but it just served to make me less stupid in the future. Or uh, at least unlikely to do the same stupid thing twice.)

Arch with i3/sway is what I'd recommend then. NixOS seems really cool, but I've heard it's very difficult. Also it's going to be much easier to test stuff out on Arch, then once you have a full setup you like, you could try Nix.

I'd recommend Neovim too, with some nice extensions. That's what I'd start with, and you can start on MacOS. I wouldn't feel at home in the terminal without it, and I think you'll really want it for setting up everything else. I'd almost say that vim or emacs is a prerequisite for something like NixOS.

Between vim and a tiling wm, you're going to have enough of a learning curve to start, I think.