The rudder on my i14 has always been stiff.
Waxing the control lines helped a little, but not enough and adding the larger rudder only made matters worse. I would often end up with a sore thumb particularly when tacking with the sail up.
After a bit of trial and error I came up with a simple modification by adding a timber handle as shown in the photos. I cut a narrow kerf on the underside of the timber handle so it would recess onto the existing handle. Also had to replace the retaining screw with a longer one.
Works great! Very responsive and no more callouses on my thumb.