How to configure/override/overlay systemd configuration

I wish set the ExecStart of a systemd module explicitly in my configuration.nix. To avoid confusing the specifics, I’m not going to go into the “why” here, I’ll ask you to trust that I have my reasons.

I tried setting = "foo";

But that fails with

The option `’ has conflicting definition values:

  1. How do I override to set my own ExecStart?
  2. Is it possible to use a similar technique (perhaps overlays) to set an ExecStart based upon configuration values, e.g., setting rsyncd’s ExecStart to "${pkgs.rsync}/bin/rsync --my-config=${configFile}";

If you have options that you want to override the value of that has been set elsewhere, then you can use lib.mkForce.