Unfortunately, Ink & Slide is not fully compatible with either of those models. I apologize for the frustrating experience, I know that's not what you wanted to hear. As for the iPad Pro, the best stylus to use would be the Apple Pencil with Photoshop Sketch or Illustrator Draw. I see you've noted that you experience bugs when using just your fingers. Please let me know what bugs you are facing when you use touch input instead of a stylus.