Ohohoho
I found a workaround! Instead of using a canvas and manually drawing the grid/notes, I got it to work with Scalable Vector Graphics. Not only is it faster than before, it also exposes notes as parts of the page instead of a single image. That means that eventually, accessibility options might be possible! Also, it will scroll as smoothly as before and will look sharper on higher resolution displays, which is nice.
And it can now import USTs as new tracks instead of replacing the entire project.
With that immensely annoying rendering problem out of the way, I can focus on more complex tasks, such as implementing pitchbends. I honestly have no idea how to accurately replicate UTAU's pitchbends, but I'll try my best and keep everyone updated on that.