Some suggestions:

Pen input (Apple Pencil) is quite choppy, because browsers tend to coalesce multiple pen events together, effectively reducing the pen's polling rate. You need to use PointerEvent.getCoalescedEvents: https://developer.mozilla.org/en-US/docs/Web/API/PointerEven.... Pen pressure support would also make signatures look a lot better.

Would also be nice to be able to draw anywhere you want. Right now I'm using Xournal++ for pdf annotation but its performance is lacking and I'm always looking for alternatives.