How to properly test a ready-made pyqt desktop application?

Question

Python

There is a ready pyqt application. It is necessary to cover the application with tests to the maximum. the first thing that comes to mind is to use pytest-qt: call the main application window, emulate button presses and analyze the results.
There is an option to test only the business logic, and mock everything related to the UI part.

Which approach is correct in terms of minimizing problems in the future when developing the application?