while centering the
rudder I recommend to slide the cable-attachment towards the end of the tiller and to fix it there by some means, for example by mounting on the tiller two hose-clamps, one before and the other aft of the cable-attachment. The more You move the attachment towards the end of the tiller, the more leverage You will obtain, and the more the
cables will be tensioned, which is what You want to achieve.
On the other hand, You should not tension the
cables too much. The rudder-wheel should move about 5 degrees before provoking an effect on the
rudder.
Your photos don't show the pulleys on the two sides of the
boat, guiding the cables towards the tiller. You should check for wear there.