I was randomly trying to look for answers, and found out, even if that was not supposed to be the answer to my question, that :
using "border-image" instead of "background-image" or "image" literally remove the pixelized stuff and SOVLES IT ONCE FOR ALL !

There is no straightforward API for this. I see several possibilities:

custom painting in QQuickPaintedItem custom painting in QQuickItem custom painting via GL shaders custom painting in QML Canvas a set of 4 animated Rectangle components, simulating the line. An ugly solution, but probably the easiest to do

Perhaps BorderImage component can be forced to do an animation like that, but I doubt it.

In C++ you register your image provider by providing a new resource prefix (say you chose "myimage") then in QML it will look like this:

source: "myimage://myborderImage"

Read Qt doc about QQuickImageProvider class, there is an example of how to use it