A bigger change would involve making thrusting also matter for the score:
You could have energy slowly deplete and value accrue during thrusting, and then color-code the speed bar for the currently optimal thrusting speed; in the green area your score rate is doubled, say. And that green area starts at the left side and slowly moves to the right. Then the best time to cum is when you are already in the green at the fastest thrusting.
This, I realize, would be much more effort to implement.