It would be helpful to understand why the design is like it is from an engineering viewpoint but without that information, why do you think it is flawed?
Agreed it seems a little clumsy and there may be a good reason for not having an additional automatic mode, we just don't know.
Fuel management, in whatever form from endurance planning to quality to refueling safety to economic and accounting aspects ETC, is so utterly fundamental to being a pilot (glider pilots may disagree of course) you can't see the procedure for the 135 being too much of a challenge.
Exactly what do you think is wrong with it?