See my comment above: I am under no delusions that cross-platform frameworks are truly cross-platform on the UI level. But, for our app, the UI layer is roughly 50% of the app. What about the core of the business logic? The code in a cross-platform framework is truly cross-platform. How do I approach that doing full native development for Android and iOS? Is there a way to create a cross-platform library?

It's called C++.

And I can tell you, from my own experiences, your backend isn't going to be as cross platform as you dream of it being because everything from networking on down is done differently on Android and iOS. Which isn't to say that you can't front-end all of that with a shim in C# or C++ or whatevs, but on mobile, the bulk of the hard stuff is UI anyways and there is no cross-platform panacea as of this writing.