Third party app development must be a nightmare, based on how much of a nightmare it is even just to install and use those apps.

But I must say, I wish other appliances would be as intuitive as the built in fitness tracking apps and they controls. Somehow it's just consistent, does the right thing, and works reliably. With only 5 buttons.

It truly is. You're forced to use Monkey-C, a homebrewed language that's probably the single worst language that anyone has to program in: https://developer.garmin.com/connect-iq/monkey-c/