I reshaped the neck a bit tonight. I made the part between the 8th fret to the heel more round and in line with the rest of the neck (starting with the rasp again) and finally I improved the shape along the complete length of the neck with a shave. It’s a lot better now, maybe still not there, but close. It’s a flat regular D shape all along right now.
I also laid out the neck attachment layout with the actual ferrules over the CAD-drawing. It’s going to look like this: