QML and JavaFX are cross-platform, XAML is cross platform via Avalonia or Xamarin.Forms.
Anyone using C++ on Windows for GUI development should use Visual C++, C++ Builder or the gcc toolchain integrated in the Qt installer.
Avalonia is at beta level at best, would you bet your startup on it? Xamarin.Forms ok, but XAML is not 100% the same everywhere, anyway. WPF is not Xamarin.Forms.
http://gluonhq.com/products/mobile/
> XAML is not 100% the same everywhere, anyway.
Hence XAML Standard,
And be incredibly shitty.
If you care about your users and their experience, you won't write apps in JS to begin with.
Anything else should take advantage of the native UI/UX tooling.
msys2 (http://www.msys2.org/) solves package management quite nicely, at least for the more popular libraries and tools.
Oh, and that one is practically designed around a hybrid C++, so if you don't like C++ (last time I tried C++ with mingw on Windows I had a horrible time installing libraries), you're out of choices.