Because I had my forks out because of changing the headstock bearing I also changed the fork oil. I measured the oil level which was 160 and 180mm of air while the book says 145mm . The oil itself looked pretty dirty with metal flakes in it (50.000km).

So I put in new oil (ca. 600 ml per fork) and set the level at 145mm. On the first ride I noticed it wasn't diving that much anymore under braking and it felt a lot firmer in flick flak turns.

So if you want a firmer front end, check your fork oil level. But unfortunately the damping in the back (especially the rebound) is still to soft of course.