…should I cut it with a square end mill in several passes and then use the same end mill to profile the side slope by tilting the dividing head by 10 deg with some more passes
While this is possible in principle, it might be very difficult in in a practical sense, because the pivot point/fulcrum of the tilt would need to be at the intersection (vertex) of the two sloping sides.
If you pivot about some other point, you then have to move the cutter in additon to the angular displacement (it is very similar to how you would cut bevel gears by the parallel depth method).
The 20 degree included angle tapered endmill suggested by Jason makes the set up a lot simpler. You can keep the spindle centred over the dividing head’s rotational axis and just offset in X or Y when using the tapered cutter. Creating a 4.5mm wide straight-sided slot will leave the tapered cutter little material to remove.
If you know of someone with a cutter grinder, it might be possible to make a horizontal milling cutter with 20 degree included angle. If you went slowly, a flycutter tool in the horizontal spindle might also work.
What is on the other end of the piece you show in the photo? Maybe making a whole new shaft with a simpler coupling would be less work than trying to make something to fit Schaublin’s original.