sometimes its all about the small things. as long as i dont have a DisplayDriver, which writes the render result direct into the Fusion framebuffer, i needed to provide some feedback about the rendering progress.
i started with a simple text output to the console and then i rembered that i saw somewhere there is an function to set the progress bar of the tool.
And here it is:
as you can see the progress bar of the Render3D will update correctly with the status of the rendering progress. now you can see how much longer you have to wait for your nice picture.