When you want to persuade Germans, focus on the quality of your argument, not on how impressively you present it. What matters is whether your reasoning is sound, your facts are correct, and your logic holds together. A plainly presented argument with strong substance will outperform a brilliantly delivered weak one. Germans evaluate the content of what you say—they listen for whether your points make sense, whether your evidence supports your claims, whether you have actually thought through the issue.
Charisma and rhetorical skill are not irrelevant, but they are secondary. If your substance is weak, no amount of presentation polish will save it. Build your case on solid foundations: good information, sound reasoning, genuine depth of understanding. The substance will carry you.