I suggest we should have the following long-term goals:

• Common formatting rules (the typical bike-shedding topics: tabs vs. spaces, where do the {} go, ...)
• Naming conventions (CamelCase vs. snake_case, filename conventions)
• Common understanding how to establish automated testing of our functionality (beyond "it compiles!")
• Common understanding what belongs into mainwindow.cpp and what should be moved out.
• Common understanding how to organize application state. (We presently sometimes use UI elements to keep state and sometimes use member variables that are treated like FORTRAN common blocks of old.)

In each of these cases, it is one step to come up with an idea what we want. Then it is a second step to convert our code base to that desired shape. That second step tends to be a journey. Depending on what we're talking about, it can be completed either by brute force in one fell swoop, or by-and-by as time progresses.