Normally, when I deal with scalability it's in a smartphone environment which has much more variableness to it than your situation but I think the same principles hold.
Generally speaking, I don't think you should try to scale the logical pixel ratio of the entire user interface. I think it should stay at 1:1 where one logical screen unit (pixel) in QML equals one logical screen unit (pixel) outside of QML.
The main reason for this is you are normally going to have at least two different aspect ratios: in your case 800x600 is 4:3 and 1280x720 is 16:9. When the aspect ratios do not match, you either have to clip content or have black bars on the sides if your UI itself chooses one or the other of those aspect ratios and tries to display full screen on both (assuming you don't want to stretch the image of the UI in either direction - which almost never makes sense for a UI).
So, what I normally do is use RowLayout and ColumnLayout with fillWidth and fillHeight set appropriately on the UI itself so that appropriate interior areas of the UI stretch to accommodate greater or lesser space as the screen size changes. It's the same principle used in web apps to accommodate various window sizes.
One great way of doing this is to build the app to your desktop environment (Mac or Windows) and just resize the window and see how your layout adapts to different screen sizes. Once you've got it laying out rationally for the range of sizes you have to deal with, then you can build for your target environment and it should run fine.
