What software do I use to create a commercial desktop GUI?

I come from an academic context and am deeply confused as to whether there is any free software available for making a GUI front-end to deliver my application. My desires are as follows:

1) I would rather work with something free. For the simple reason that I'd rather this little project didn't cost me hundreds/thousands of dollars per month up front. I just wanted to sell a little application for a few bucks for lolz.

2) I would rather work with something that doesn't force me to disclose all back-end code.

3) I would rather work with something fast/non-bloated, cross-platform (Windows and MacOS will do), and not obviously ugly to the average consumer (since I want them to buy it). In fact, prettiness is really quite high on my list of priorities because, well, why not?

I couldn't quite understand what exactly my obligations were when using, say, the open source license of QT. I'm happy for customers to see, say, the source code for my GUI logic. I'm not happy for them to see my back-end data logic.

Happy to learn Java or Python or C++ or whatever language is best suited to this task. Just don't want to accidentally get myself entangled with strange license obligations.