An app called Astropad allows you to access any software you run on your computer and use the iPad like a Cintiq. The actual software on the computer, OpenToonz in this case, does not get installed on the iPad so you do not need to look for iPad/iOS software. Astropad basically captures a section of the display and reproduces it on the iPad screen. If the software can be used to draw at all the Apple Pencil can be used as the input device. So far the only difference I have experienced is with Autodesk Graphic. When I draw a stroke there is a brief moment when it is rasterized then it updates to a vector on the iPad. It is so fast without a delay anywhere except in the art quality so you do not get tripped like a hiccup might cause. When I use the iOS version of Autodesck Graphic there is no delay at all. But as you know there is no iOS version of OpenToonz. I am able to use Toon Boom Harmony Premium, Photoshop, Illustrator, etc. flawlessly with the Apple Pencil/iPad Pro/Astropad setup.