*Reduce* shower pressure?
Usually people want "less pressure" because the showerhead is laying out an unusually harsh spray.  I've had shower-massage heads for the last 20 years, that are switchable between any of up to a dozen  patterns, and I've found 1 or 2 are always harsher than the others and give that "getting pummeled" feeling but not in a good way.  

So the first stop, I would say, is to change the showerhead, which I gather came with the apartment.  

The purpose of the single knob is to provide thermostatic control so you can set it to a temperature and it will stay there regardless of pressure fluctuations. That's a nice feature, but their reason for doing it is to obtain anti-scald protection, which is easy once the thermostatic mechanism is present.  They care about anti-scald because they run the hot water temp high enough either a) to provide anti-legionella protection, or b) also work as district heating, which runs quite hot.