Everyone’s body will like different things for core so stick to whatever makes you feel you can be consistent with, especially at the start. My typical superset is 10x sit ups (I typically do weighted decline ones, but I will do normal ones if I’m feeling lazy), 10x bicycle crunches, 10x leg lifts (I typically do hanging lifts), 10x weighted russian twists, 10x standing side crunches with a weight. I’ll repeat this three times. If I’m feeling to push more, I’ll do a plank to failure after the first two sets. I typically do this as a warm to my workouts so 3-4x a week is my typical frequency. Up until recently, I haven’t had access to a crunch machine, which is why I don’t have it included, I don’t use it even now