If you don't care for the speakers, then get iPad Air 3.
However, I do recommend the iPad Pro because the 4 way speakers audio quality is significantly better and it's stereo (huge PLUS!). Actually, that's the only reason why I would recommend the iPad Pro.
As for future proof, the iPad Pro 2017 is already pretty fast. For drawing purposes it's definitely more than powerful enough.
When drawing, you're probably not going to notice the 120Hz ProMotion because I doubt you will be drawing so fast to realise that ProMotion happening. There's a reason why all Youtube reviews show off ProMotion by swiping the icons on the home screen. LOL. When you're drawing, you're just moving your Apple Pencil across a small distance, not like the icons traveling across huge distance or you scrolling web pages that fast.