Considering that you do have a healthy diet and that you are including intelligent cardio (love that Joy!) into your workouts, I would suggest incorporating weights along with your abdominal exercises.
I have found that by adding weights to your exercises, you tax your muscles with more emphasis and speed up your results plus it is less boring.
Less boring because you get to use a variety of different equipment such as dumbbells, weight plates, medicine balls, barbells, etc…..
I would include weighted saxon bends, bicycle crunches, and russian twists.
I personally follow up with the best abdominal strength exercises (in my opinion)
the forward and side plank.
If you subscribe to the notion that we all have similar abdominal musculature underneath, and the real issue is how much body fat covers them, then I say that the best “exercise” for this goal is aerobic exercise. While I TOTALLY agree that diet is extremely important to this goal as well, I don’t consider diet an exercise so am not including that in my answer. That being said, the two-headed monster of aerobic exercise and diet will help those abs that you already have shine through.