The lower belly area is a funny space, TRUTH, I wasn't really able to understand it until I learned a crap ton about NUTRITION AND METABOLISM. However, if you want to get a sense of what I do, here are some TRX core ideas.
Here are some simple baselines I use:
I usually try and do 4-5 exercises each training session.
If I am learning I try to do 3 sets of 10
If I am trying to build I do 4 sets of 12, then build to 4 sets of 15
Eventually with strength and mastery I do 4 sets of 20