I have no answer, only a work-around.
The documentation says a script can be an autoload, and I have used a script as a collection of static functions. The documentation stays silent on what, exactly, is a script as an autoload, what and when variables are evaluated, and what is available to a
The simple work around is to create a new scene with just a node and that attached script. Make the new scene an autoload. Being a scene, it will follow the normal lifecycle and be made ready before any of your written scenes.