I have never had any problems with my steering control cables. I suspect that they are original. I have used them for 25 years without any problems. They are still in good condition with no broken strands.
There may be an issue in that you say that you lock the helm when on a swinging mooring and that the rudder is raised. My understanding has always been that you should NOT do that, particularly if the blade is raised. With the rudder raised natural movement of the boat can place an enormous strain on the system – such as when the wind opposes the tide. A shock cord which allowed the tiller to move would be much kinder to the rudder system.
I don’t know that this would be the cause of your problems – but it might have a bearing on the problem.
On the other hand I have frequently had to replace the uphaul and downhaul wires which pass over extremely small sheaves.