Hmm apart from distribution (which is a pain to set up) I still don't think you can beat QtWidgets and QtCreator for simplicity of getting a professional GUI. It has a form editor that actually works. I think maybe the only one I've ever used. Then you can pretty much just click on buttons and add event handlers. Very easy to get going and it scales very far.

There are a few downsides... there are better options than C++ these days (Rust most obviously), QtWidgets is in maintenance mode, it's a bit of a pain to make an installer from your app, and it doesn't really support modern styling.

But I'd still pick it over this in a heartbeat.