You can install Nix package manager on any system, but the packages will be separate from system packages. You can try it and switch to full NixOS if you like it

Yes. I think it's a good way to get started with Nix.

e.g. with nix installed, you'll be able to use the nix-shell to get the development dependencies without having to install system wide, if a shell.nix or flake.nix has been written for it. (e.g. for repos like https://github.com/helix-editor/helix).

You'll be able to use the `nix shell` command to try out a program without needing it installed system wide. Or you can figure out how to write your own nix-shells etc.

I think Home Manager is worth trying only once you're otherwise familiar with Nix (or if you find a good config to copy-paste from, though), since Nix's error messages and debugging still aren't great.

If you are talking about provisioning dot files and command line applications you want to have available on all you machines then take a look at home-manager.

https://github.com/nix-community/home-manager

I use it on NixOS. It should absolutely work on other distros unfortunately I don't have any first hand experience with that or with what the best way is to set it up on other distos (I think there are few options for how to do it).

Did you ever try to patch a software in debian or create your own package? Debian had no native and easy way to do that unless nix. If you have a standard cmake project you can be done in 10 minutes packaging it. Also applying patches is very easy with overlays while also keeping your software up to date and propogate the changes through the dependency chain.