I think that the order of things loaded differs a wee bit with the 2 platforms (at least it seemed to in the past.)
On Windows, I believe that just after all the extensions are loaded (and their menus and toolbars built,) the SU core calls any AppObserver#expectsStartupModelNotifications callbacks to find out if it should then call the appropriate other “model” callback. So this one can be used as a signal that the extension load cycle has ended, and that there is a valid model object ready.
Your test script would need to be the very first loaded in the “Plugins” folder.