JavaFX doesn't run on iOS. While your Electron app will be tweaked to make a Cordova app that runs on iOS.
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.
My point of view is that if it is to be done with the Web stack, then by all means it should be delivered to whatever browser the user likes to use on their platform.
Anything else should take advantage of the native UI/UX tooling.
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.