I'm having similar steering problems where the steering is extremely stiff, and I've had two events of being out on the water where the steering line slipped from the tensioning screw, and the boat became uncontrollable. I've tightened the screw to the point that the line is now frayed, but there still seems to be a tendency to slip. The rudder is all of the way down. This is unacceptable from a safety perspective, and I don't trust the boat anymore. The steering mechanism is just too complicated and prone to failure. Is there an option to get a normal tiller?
Your issue is not design it is technique... an possibly one or two knots needed.
Pull the rudder down line HARD and CLEAT. That holds the rudder down. If you stand behind the boat and pull back on the rudder and it easily moves up a bit. You have not pulled the down line hard enough or it slipped before being cleated. If the rudder slips aft while sailing, you will experience stiff and difficult steerage which puts a strain on the adjustment lines and connections.
After adjusting the steering lines at the rudder. Use the excess line ends to tie a couple half hitches. This locks the line it that position and is the way boats are delivered from the factory.