Hi,
The last Qt Design Studio version, which has still native Qt 5 support, is Qt Design Studio 4.1. Therefore this version is still available in the Maintenance Tool.
Make sure to choose Qt 5 for the imports and for the kit.
In this case, the Qt 5 based QtGraphicalEffects are fully available.
In Qt 6 only a limited version (import Qt5Compat.GraphicalEffects) is available for compatibility.