I think NixOS is awesome, but it certainly doesn’t offer “access to (basically) all Linux-capable software, no matter from what repo.” - at least not natively.
I don’t quite agree with this. In NixOS you can write custom expressions that fetch software from any source, and stitch them into your configuration as first-class packages. So you do get access to all Linux-capable software natively, but not necessarily easily. (There is a learning curve to packaging stuff yourself.)
I use this process to bring nightly releases of neovim and nushell into my reproducible config. Ok, I do use flakes that other people published for building those projects, which is a bit like installing from a community PPA. But when I wanted to install Niri, a very new window manager I wrote the package and NixOS module expressions all by myself!