OnShape is Free for up to 10 designs and 100gb with no restrestriction on resoluion of the export.
3D printing is a hobby for many which at some point might become a source of income but until then most hobby are not willing to pay $30/year for a tool when so many are free.
Auto desk fusion 360 is also free for non professionals"Free for students, enthusiasts, hobbyists, and startups", freecad, blender, openscad, tinkercad and more. These tools unfortunately do not run on my iPad hence the search for an app like yours. I am willing to support the developers of an app I find of value but not $100 or even $30/year... Sorry but since the 3D printing /maker fair advent your market has become more competitive.