This is my suggestion on how the advanced "Customize Toolbar" UI could look. It could also be combined with the standard buttons. Next to each submenu or menu action there is a button that you can drag to the toolbar to add it. You could also search for available commands. I think this view is simple and understandable for everybody.
Apologies for the bad quality of the mockup, first time I tried Figma.
Edit: Somewhere there should be a "Done button" when the user is finished. As well as indicators for items already added, and indicators for toolbar spaces.