system-manager VS templates

I meant that both /etc and systemd management are possible via nix on non-nixos, using a flake that has system-manager (see https://github.com/numtide/system-manager) as an input

When I changed self.lib to sysmgr.lib (my intention being to try to use https://github.com/numtide/system-manager/blob/master/nix/lib.nix which actually has makeSystemConfig defined), I ended up with this error:

I haven't used it but system-manager might help https://github.com/numtide/system-manager

actually there is system-manager for managing a lot of system-level config including systemd stuff

None at all, I recommend Nix package manager and home-manager. As time progresses and system-manager grows in popularity I could start recommending that too.

Unless you want to handwire something together or use another higher level function from some 3rd party library, your want to use this function. It is basically the entrypoint to a flake based nixos system configuration. See https://github.com/NixOS/templates/blob/master/simple-container/flake.nix for an example.

There are some templates on the official repository. https://github.com/NixOS/templates You can find others on github.

Don’t forget about the official templates. Built right into the CLI https://github.com/NixOS/templates

I am new to nix, and even newer when it comes to using flakes. I was trying out the following simple flake (based on this template), which just terminates with a segmentation fault whenever I use --impure flag (e.g. nix develop --impure)

You would probably want to use flakes. There is NixOS/templates repo with flake templates. Take a look at go-hello one.

You should consider adding this as a template to https://github.com/nixos/templates.