FLTK is great at being fast and light, that’s about it. It’s kind of cumbersome to use but honestly does what it says on the tin. Highly recommend for smaller use cases but can’t imagine using on a large project. I used the rust bindings which are well maintained.
Yeah for my work, legacy Win32/WinForms/WPF codebases tools are kept maintained as-is, but new tools are usually written in PySide6 (QtWidgets or QtQuick) and it's worked out really well (other than bundling/distribution being tricky for big apps)
You will need it, because since Windows Vista, most new APIs are COM based, as they redid Longhorn ideas in C++ instead of .NET, and WinRT also builds upon it.
Classical Win32 C API surface, with some exceptions, is mostly stuck in Windows XP view of the world.