Calm down. I've been building medical device software for close to two decades and the level of engineering in this device is at least what I see on multi-million dollar projects that are going for FDA approval.

Medical Design process basically exists (oversimplifying of course) because the engineers are not the users. It's entirely possible for one person to build something that far exceeds the quality and performance of an off the shelf device simply because they care to do a good job and have constant feedback on how it works.

There's a lot to be said for scratching your own itch!