We were told Natsu trained to learn how to use Igneel's power, not to master dragon force. DF is really something that Natsu use instinctively when he gets pushed to his limits. Are you saying Erza would beat Mard Geer if she was in Natsu's shoes? Again, why would Franmalth be weaker than Hades if Franmalth literally absorbed Hades? Franmalth should be as strong as Hades without his heart. He can also absorb magical power, so Laxus and Erza would only beat him if they did what Natsu did with the rock.
What attack from August did Gildarts really tank? August didn't throw any big nukes at Gildarts, except for Ars Magia which Gildarts probably wouldn't have tanked. Natsu didn't negate the attack so that only he could survive. He negated it so that Happy, Lucy, and Mest were relatively unharmed as well.
Natsu has managed to land shots against Zeref. Gildarts never fought Jacob. You can see the marks on August after he emerges from the rubble. August is coloured darkly so it's hard to see them clearly, and they don't really show up on subsequent panels. Still, Zeref's scratches are very minor. It's silly to suggest that August is on a whole different level compared to Zeref based off a few bruises. Zeref looks way more beat up after fighting Igneel-powered Natsu. Just look at their conditions side by side. You don't think the scratches on Zeref had any blood?