Series Landies steer well when set up correctly. However it is a long process to get them right and adjust/replace worn components. A few points to be going on with, check the steering box to chassis mountings are tight they can come loose. Make sure there is oil in the relay, remove 2 screws in the top one to fill the other to bleed air. Ensure the relay arm is set correctly, look at the manual 89/91 degrees. Do all the usual checks on rod ends to eliminate wear.
When all the above is done check the swivels, see the manual for correct pull with a spring balance. I always go for around 12/14 pound pull if the seals are left in place which is more than the manual states without seals.
When that lot is done finally check the tracking, the tracking makes a big difference if wrong. None of the work is hard to do but they will never feel like a modern car, if they did they would be awful off road.