Slint is not a replacement for Qt. Slint only provides a tiny fraction of what Qt has to offer https://doc.qt.io/qt-6/modules-cpp.html
QtSerialPort (https://github.com/search?q=include+QSerialPort&type=code)
Qt3D (https://github.com/search?q=include+Qt3D&type=code)
QtWebSockets (https://github.com/search?q=include+Qt3D&type=code)
QtSensors (https://github.com/search?q=include+qtsensors&type=code)
Qt SCXML (https://github.com/search?q=include+qstatemachine&type=code)
etc etc..
Qt's value is as an app framework where you can assume interoperation across the same async runtime and enjoy a cohesive set of APIs for all your app's needs.