Eh. I mean it's certainly a smaller curve than other "hard" distros like Arch or Gentoo, and there really isn't one at all since the installer does most of the complicated stuff for you.
Would I recommend it to beginners? Probably not as they wouldn't be willing to do any reading, configuring, or time sinking at all.
However, for this use case of building solutions by an experienced Linux user, the 30 min to an hour of learning is really not a lot when it would save a ton of time down the line. It's not like you need to be a nix lang or nixos expert to use it effectively
I mostly agree with this, I have it on my laptop. Took an hour or two to learn it, used a live image from the website just like any distro. Not for beginners, but someone that is used to arch, after you rtfm it's fine.
I see more and more people mentionning NixOS, until I read your message I thought it'd be more complicated than that to use it.
But I have a beginner question: do the Nix repositories contain many packages that you'd want, or do you find yourself installing stuff manually?
That's actually one of its selling points. 80k packages. It's more than the AUR (or any other package manager, for that matter).
I've only had 3 programs not be available so far: a tool someone made for RGB set up on MSI laptops (somewhat niche tool) and Slippi & Project+ which were only available as AppImages that for some reason wouldn't run and need their own environment (other AppImages seem to work fine)
Very rarely will something not be available, and even then, someone has probably already figured out how to install it; it's just not in the main repo, so a quick internet search will remedy it without you having to do any thinking yourself. I didn't solve the Slippi thing myself.