@Beatrix W Try "Clean Code" by Robert Martin. You should have smaller classes with a clearly defined job. The window and the functionality of your app should be decoupled.
One standard way is to have the MVC pattern - model view controller. I found the controller not to be of much use in Xojo. My main window delegates its calls back to the model, which calls the different classes. Then the model talks back to the window.
The problem is I'm not skilled enough and I understand only partially you said. The approach I'm trying is to group in a class o module all procedures that share something, or deal with a part of the window's task, that's works, but, as I use large names for properties, methods, etc., to be as clear as possible for myself to understand the code, some calls o references are very long. Just the option to define folders inside the window/methods will help a lot.