I have had some success helping beginners in short kayaks who are having trouble going straight. I offer the same advice as you.

The way I explain it is to tell them to only do the front half of the stroke. Don't do the back half of the stroke. That is what makes the kayak turn.

I also tell them not to put in so much effort. Most beginners try to cope with the problem using brute force. That only makes the problem worse. Very gentle strokes will make the kayak go straighter.

Pausing too long between the end of one stroke and the start of the next gives to kayak time to turn beyond the point where it can be recovered.

So, the answer is fast, very short, very gentle strokes. Most beginners struggling with a kayak with poor tracking seem to instinctively do the exact opposite. As the paddler learns control, the strokes can become longer and more powerful.

A short kayak is going to be slow. Trying to make it go fast generally only makes it hard to control.

A change in technique can make an enormous difference and it costs nothing to apply it.