What kind if items do you create as graphics items?
Grids and Ellipses are created as graphics item and in future vendor company will also create some items as graphics Item.

It might be possible to just make custom declarative items and create all items from QML.
In Qt 4.8.2 we used QDeclalativeItem to create some objects in QML and added it to QGraphicsScene. But now in Qt 5.7 how will we create it?

So currently you have a QDeclarativeView as an item in your QGraphicsScene?
No, We are not using QDeclarativeView. We used QQuickWidget to access Item form QML file and added it to QGraphicsScene.