The folks that made Astropad have a new version they are calling Astropad Studio:

"Astropad Studio turns your iPad Pro into a completely customizable graphics tablet. It's our highest performance tool for the most demanding creative work."

It's really good. I've used the previous version and it worked pretty well. This new version works amazingly well. The display updates faster than what I would have thought possible, and it looks like there's a bunch of customization you can do. It's worth checking out if you've used tablets in the past.

It works great with Acorn too.