A couple of Python libraries that you might consider using to simplify the creation of a GUI with Python's native Tkinter UI library are tklayout and tkpane. These libraries do not provide a GUI form designer, or an IDE, but they simplify the coding of UI interfaces.
The tklayout library allows you to specify a high-level description of the spatial relationships between UI elements, and then automatically builds the Tkinter frames for that structure.
The tkpane library encapsulates Tkinter widgets in objects (panes) that have methods that push and pull data between different panes. With a visual form designer you ordinarily have to code all the data-handling operations yourself, but the tkpane library eliminates much of that coding through the use of methods that describe the data dependencies between panes. After the dependencies are described, data are automatically passed between panes as the user interacts with the widgets.
Disclaimer: I wrote tklayout and tkpane.