To correct the first problem the rudder blade needs to be moved forward with
respect to the axis of rotation. By moving the blade forward the rudder is more
balanced and the pull on the tiller is reduced.
To correct the second problem the rudder needs to be larger or shaped better so as not to stall. The rudder shown in these pictures was made to be 2 1/2" further forward than the stock rudder. It was also shaped to be a better foil shape and therefore not stall as easily. It is also 4" longer than the original. I am still experimenting with the length. Since the rudder is made of wood it is hard to get down because it floats. I am going to add lead to it solve this problem.
In the lower photo I am holding the old rudder in front of the new one. You can see that the new one is 2 1/2inches further forward and extends 4 inches deeper. This gives much better control.