I am going to show youimum of your ab workouts.
Some might tell you that you only need two exercises total in order to build a muscle group. I disagree. I don’t think anyone should be limited to picking just two exercises for a muscle group because each one often has multiple muscle heads as well as multiple functions that can be targeted individually through exercise selection.
In the case of the abdominal muscles with a wide variety of exercises, namely the rectus abdominis and the oblique muscles; you need to train them through not only their functions, but you also need to target the different areas of the core muscles themselves. Believe it or not, you can actually preferentially influence the upper and lower portion of the abdominal muscles based on where you initiate the movement from. And don't forget to include rotation to target the obliques.
Another thing to note is that instead of choosing just two exercises to build a six pack and strong core muscles, you will be choosing two movement patterns with 3 simple exercises (one for each ability level). Not every ab workout needs to include both of these movements, but a complete training plan for your back will at least build off them or include them at some point.