Complexity, Accuracy, and Fluency are used to quantify progress in second language acquisition. I found in this Prezi presentation by Julia Jensen some definitions for syntactic complexity in terms of words/unit (where unit can be clause, T-unit, or sentence).
What are other measures for syntactic complexity? The measures should be applicable to English and German (they do not need to fit every language of the world).
Measuring the range of the vocabulary used in a grammatically/syntactically correct way (lexical resource) could be added here. Or even the grammatical range is another such quantifier. But, what you've already mentioned mostly sums it up. However, if you're writing an essay, just be careful not to overdo it. In writing, the style, or tone, also plays a very important role. More complexity does not always imply more marks. Hope this helps! :)
