I'm having this this issue too, and it's SUPER frustrating. I know that Animate CC's brush tool is super busted/janky right now, but I also feel like part of the issue here is with Astropad's default pressure settings. As you said in your original post, I too don't really get this issue when using my Cintiq 13HD. I had to bust out my 13HD last night because I was getting so frustrated and it bums me out, because I SO WANT to ditch it for a stricktly iPad Pro/Astropad workflow.
I really think adding a customizable pressure curve editor is necessary, it seems like a really basic feature and I'm kind of puzzled as to why it's not included. Also, I find myself pressing way harder than I'd like to or feel I should in other programs like Photoshop too, so for me this isn't just an issue with Animate either.