Hi Breezy!
On my stylus, I can set one of the buttons to "Shift". But that doesn't work anyway, because I can't press both buttons at a time, and you will always need to press two buttons to zoom.
I think the best solution for you would be to use the "Canvas Positioner" (Screenshot). If you click on that button up there in the menu bar, you will get this round panel. If you drag with the sylus in the upper field, I marked with an arrow, you can zoom the canvas. You can even zoom in steps, rotate and move the canvas with this panel. Just test it out.