Today I spent another hour sanding in the radius. I switched to higher quality sandpaper (not very good, but at least better than what I was using before) and the result is stunning. In contrast to the previous sanding session, the fret slots and the complete nut slot actually filled up with dust in minutes. Whereas after an hour of sanding last time the centers of all slots where still completely clean.. I’m now almost finished with the 80 grit, got a small area around the higher frets that still needs to be rounded, but this sure is a motivator.

Some images to go with the progress:

You can see this sandpaper is much more effective, judged by the amount of dust piling up around the neck. It also leaves quite visible marks, so I have to switch to a higher grit soon.