I use a Chromebook and I know that the ChromeOS lacks support for third parties to access the USI pen's side buttons, but I think we can circumvent this issue with a donut popup. It would work for every stylus pen as well.

The donut popup would open by holding down the pen for a couple of instants, then, you could swipe up, down, left and right on the screen to choose a tool to use like pen, selection, eraser or text.

I think this idea is simple, but powerful as it would not only circumvent the ChromeOS's limitations, but would also add a new feature that could be very handy for all the users with stylus. As it's a note-taking app, you're all the time writing down and erasing notes, so this could make the process more practical and intuitive.