If you want UI, there currently is no good solution. In my opinion, AppKit/UIKit is the only well-designed UI with any currency at the moment, but as you point out, it is not cross platform. If you're okay with using Dart, Flutter might be an option. Qt is well-designed, but last I worked with it they were embracing JavaScript, and why would you want JavaScript if you're using a C++ library? C++ is too philosophically opposed to JS. But Qt just feels slightly foreign on a Mac, anyway. GTK is in C, which is not really how I want to use an object-oriented design, plus its cross-platform feels buggy. And HTML is just simply not designed for user interface; writing a UI in HTML + JavaScript is basically equivalent to writing an app in MS Word macros. Plus, it's hard to take the NPM ecosystem seriously when you're old-skool enough to think that left-pad is the equivalent of fizz-buzz and putting it in a package should not even cross one's mind, let alone actually using said package.

I'm writing a cross-platform UI library (C++, but I hope to add Swift, Python, and maybe Rust bindings), because I'm in the same boat: https://github.com/eightbrains/uitk. It is usable, but still under development, and while the .h files are fairly heavily documented, there are no examples yet.