QT, writing C++, or both?

Qt.

Paying for a good technology can be cheaper in the long run if you save development time.

Only until the price gets jacked up beyond what you can afford, and then you’re scrambling to rewrite your entire application to use something else that’s still affordable.

And sure, developing in C++ is more expensive than JavaScript, because you can’t let cheap web code monkeys do it.

An awful lot of code is written in C++, so I’m not sure that was ever a serious constraint.

I think I made it quite clear, that I set the scope for the desktop. There are several.

Sure, if we’re targeting desktop only, then there are lots of options: GTK, wxWidgets, Swing…

But what does it matter? You can’t ignore mobile in 2023.