I too develop only desktop Qt/C++ applications. I stopped with Qt 5.15.3 because Qt's licensing model for Qt 6 was unacceptable for me. The set of UI widgets in that release are sufficient for my needs, and if I need something else, I look to KDAB, KDE Frameworks, or write my own.

I'm not all that worried if my applications look a bit old-fashioned. The market for which I write my applications isn't impressed by gee whiz UI features, they want usability and more importantly the results that the software produces.