Istvan, I am an Architect in the States, a Mac expert and very experienced with CAD and Modeling applications, namely Vectorworks and Cinema 4D (OSX / Mac OS). Apparently there is a technical difference with the iPad Pro versus the iPad Air 2.
I did see one video where someone was using a stylus like the one I have (MEKO) though I will assume it was an iPad pro they were using. Therefore an Apple pencil is preferred though not mandatory. The only response I get is the background grid moves when I use the stylus and does not allow for the creation of any content whatsoever. There must be a technological distinction with the iPad Pro that is not present in the iPad Air.
Honestly, I find your membership business model a bit troubling as well. Even if I were to purchase an iPad Pro and Apple Pencil, I am not sure I would be willing to pay you annually or monthly for access to your app. I would prefer in-app purchases instead that extends features. I realize that you are targeting a smaller market by so did SketchUp when they first came out and now it’s being used by many millions of non Architects and Engineers since Google purchased it.
I currently use several different drawing apps including Morphello, 53, Concepts and Procreate. I’m certain that the iPad Air2 could easily provide the computing power needed to run your app.