Dale,
This may not apply but often when a boat doesn't do as it seems it ought, its in the cockpit problems. We built a boat that didn't do anything we wanted it to. Like yours it was roomy cockpit. When we added padding to the hips and a saddle horn it was different boat. I'd start with padding see if it makes a difference.

Second idea: I recently rented a current designs boat. That had an adjustable skeg. It was a small triagular fin. A bicycle brake cable attached to a barrel on a dowel slide mechanism put it up and down. IT was the difference between straight and rover all over. For what its worth I decided to play with it skeg
up and added padding. It did the same thing improved the fine motor adjsutments that keep your boat heading in the right direction. However, she still needed a bit of skeg.

I'll agree with the others that a skeg is better the na ruddder in this case. Rudders in rough water spend a lot of time out of the water. Sounds like a great ! play boat

Good luck Tweaking
!Russ