Hello and thank you for your question.
You seem to be quite fit and the thickness of the fat pad on the stomach seems to be less than 1 inch ( bear in mind when you pinch you have twice the skin and twice the fat between your fingers)
We do not have a back picture and 360 lipo usually adresses the back rolls as well!
So any where between 3 and 5 liters of aspiration fluid I think is a fair approximation.
The question of the flat stomach has to be assessed together with the position of your hips / bassin / lumbar tilt... etc. all this to tell you that lipo can reduce the thickness of the pinch but not correct the posture: only abdominal muscle tightening can do that: training or abdominoplasty!
As abdominoplasty seems to be far from a reasonable indication this leaves you with the ... gym... even after the lipo!