You have a deep V single.
There is a chance that your boat is fine, but maybe you have some slop in your steering. For some reason I doubt it. (That it has a problem)
Under the best circumstances, deep V single screw boats wander at slow speeds.
I'm surprised that your 29 wellcraft is that bad, because length helps.
I used to have a 21 wellcraft deep V nova single and it wandered bad.
I thought maybe it was my bad skeg, so I had a new one welded on.
It didn't help.
Later I found out that this is not unusual. With practice, you can get it minimized by not over correcting. I found that when I reacted slower and less, the wandering was reduced. By the time I got rid of the boat I could almost get it to go straight(ish) in a no wake.