As I mentioned in PM, for drawing one possible library would be Fabric.js
The advantage of it is that it's JavaScript, so the same implementation could be used in all platforms. So if you get it working for iPad, without extra effort it will also work on Android, but also on Windows, macOS and Linux. Of course that's assuming it can work at all - in particular I'm not sure how the library works in mobile devices, with pens. Could be worth giving it a try anyway.