I am torn about backing off to focusing on first getting the UX (which is really "the thing") right, and just emitting a MIDI MPE stream. That obviously has some other benefits but there were things I was keen on having low-level control over... but I can get over that maybe...
Ambivalent also about target—really, the iPad is the ideal and canonical target, in terms of what it can and the UX it affords... but I have this possibly unfounded sense that the amount a coding copilot can assist me is somewhat limited inside the Apple ecosystem... :/
EDIT: oh and is your engine something I can look at (or license or...)? :)