Hey Tobi,
What you need couldn’t be done using the options because though centering the dropdown would look nice, if the menu with dropdown is the last menu item, it would be cut-off that’s why the element does not support such a layout.

Because there’s no option for this, custom CSS would be required. I’ll give you an example but please note that we cannot provide samples all the time as custom coding or development is outside the scope of our theme support.
You will need to learn CSS on your own and also learn how to use the browser’s element inspector so you could see the CSS selectors you’re going to use.
The following sample code should do the job when added in the Navigation Inline element’s Element CSS.
$el #menu-item-15 .x-dropdown {
width: 220px;
}
$el .x-dropdown {
left: 50% !important;
transform: translateX(-50%);
}
If you’re interested to learn the parts of the code, we can answer that in One.
Hope that helps and thank you for understanding.