Paddling a kayak in a straight line is not a simple matter nor is it easy to describe how to do it. In that sense, it's a little like describing how to ride a bicycle. There is an element of trial and error while you learn what "right" feels like. Part of the reason for this is that we never paddle in a steady-state environment. Something is always changing. Some of the forces that affect us are wind, current, paddling technique, and boat loading.

I suspect that boat loading may have been at the root of many of your directional problems. A 20 pound pack on the deck in front of the cockpit (and the Guillemot has a long cockpit to begin with) would make a significant difference in the fore/aft center of gravity. With the bow loaded too heavily, and the stern too lightly, the boat is going to want to keep turning once it is deflected from a straight course.

This means that any flaws in paddling technique, like a stronger stroke on one side or the other, are going to be magnified.

A tangential thought is that you need to rethink what gear you are carrying on deck. About 19 pounds need to be moved to the inside of the hatches and cockpit.

And please be careful and take adequate precautions when paddling in cold water and challenging conditions.