I would like to use Apple's SF Mono font with nerd-fonts patches, but due to the license, it's not readily available.

This flake allows patching any¹ system font and using it with nix-darwin.

Patching is performed by nerd-fonts. It only works on 1 subfamily, so I added a subfamily parameter to extract the subfamily from the TTF with multiple variations. If your font doesn't contain any variations (fvar table), you can omit this parameter.

Add flake

{
  inputs = {
    # ...
    font-patcher.url = "github:libc/nix-nerd-fonts-patcher";
  };

  # ...
}

Then use nix-darwin fonts module:

{ config, inputs, pkgs, system, ... }: {
  config.fonts = {
    fontDir.enable = true;
    fonts = [
      (inputs.font-patcher.lib.patchFont {
        font = /System/Library/Fonts/SFNSMono.ttf;
        name = "SF Mono Nerd Font - Regular";
        subfamily = "Regular";
        inherit system;
      })
    ];
  };
}