In reply to @bobvanderlinden_:matrix.org
Hmm, at the moment, mine is 18M (/boot/kernels/9cvnrh9wh6r707klrv7aawl8zm8w22rs-initrd-linux-5.15.27-initrd)

basically, it creates a submodule for each of the unit-types (services, mounts, etc). The submodule has:

{
  options = {
    ${type} = mkOption {
      ...
    };
  };
  config = {
    units = convertToUnit config.${type};
  };
}

That means that such modules need to be merged to get to a systemd module. I'm not sure if nixos config is capable of this or whether I should just write it out, like systemd.nix and systemd-user.nix also do.

Is that pr-reusable-systemd-module branch (refactor vs reusable) the one you're actually planning to try to get merged?

Yes, though pr-refactor-systemd-module were the commits that I am confident that it will get merged in a form simliar to what it currently is.
pr-reusable-systemd-module might be taking things 'too far' in terms of reusabability. Even though we can use this in 3 places, they way it is using modules and merging will have a higher probability that a reviewer will reject the change.