Flutter is not an alternative to Qt, not even close. Not to speak about C++ compatibility. If anything, things like GTK+, WxWidgets, EFL are on the same page, albeit less polished and advanced.
Depends on what your requirements are. If it "modern" design (let's not start the millionth discussion on what native/modern design is here), that is also supported on mobile/web, then I would say go with flutter as UI and something like rust as your main language to write your business logic in [1]. You can even use regular c++ with their FFI [2].
