Hello everyone,
I asked the Affinity Designer Forum how to improve the workflow to optimize my export due to my problem with the export. Well, it turns out that the origin point in a document is always at the bottom left edge. This cannot be changed manually. It's important to note that if you have a file with multiple artboards like I do, the origin point of the first artboard created is in the bottom left corner, and this applies to all graphics throughout the document.
My workflow now looks like this: I design all the graphics in my master document. If I want to animate a graphic in Spine, I select it, copy it, create a new document with "Ctrl+N" and paste the graphic. I then select the artboard tool and create an artboard by selecting it in the bar at the top. This will give you exactly the size of the selected graphic. In Spine you then have to place the graphic in the middle, as already shown in this discussion.
Unfortunately, as far as I know, this is the best workflow. However, if I find out anything new, I will let you know.