With Windows or older Mac OS versions you could just change the names of the loader rb files to put them in alphabetical order to match the order you want for the menu item texts. An alternative that could work in many cases would be to edit the menu item text to match the natural loading order. The downside of this is that you have to deal with manually deleting older versions and renaming newer ones when there are updates to the extensions. Some extensions get updated often enough that I would prefer to just remember where they fall in the menus naturally.
On High Sierra, as Fredo mentioned, alphabetical order doesn’t necessarily equate to loading order.