As someone with a professional background in consumer hardware (and music production) my first thought is always "how can I solve this problem without hardware" first. Don't get me wrong--hardware is fun--but manufacturing and selling consumer hardware is brutal from a business perspective (of course fun in its own masochistic way). With hardware, you're dealing with annoying cashflow issues unless you can easily build to order (money tied up in inventory or even the supplies), supply chain management, inventory management overhead, logistics and fulfillment overhead, additional regulatory overhead.
My understanding from OP's product is that there is an inherent assumption that this is targeted towards people dabbling on standalone digital pianos that also typically have built-in synthesis/sound modules.
Anyway, I'm just giving my reasons for why wouldn't you start with a software approach to answer your question. Putting all that aside, OP's solution is really cool and, independent of the decision to go custom hardware, it seems like a great product solution for the problem it's solving.
