You do not need a definite or indefinite article when referring to certain roles in English! I've just now realised. It WOULD sound odd to say "The man is still teacher." But any powerful position sounds fine. "He is still king! He is still colonel. She is still president." Weird.
It is cases where the position is "inherently definite", that is, where in typical uses, within the context of the conversation, there can be only one entity so described so the name of the occupation can stand in as the name of the person.
One of the ways languages that mark definiteness differ is in what they do with inherently definite phrases and what the lack of any marking signifies. In English the unmarked noun is typically understood to be definite, so "I am janitor" sounds weird in English unless you are saying that there is only one janitor and you are he, whereas in German it is typically understood to be indefinite, so this is the usual way to say you are a janitor, this is your occupation.
It really doesn't matter if there's "no reason to use the zero article". Native speakers say this all the time, and it sounds and is perfectly grammatical. In fact, I personally think there is a difference between the two: "He is still the mayor" would suggest the mayorship as the topic, and "he is still mayor" suggests more to me that he is the topic.
on top of that<sub>/</sub> weiterhin this does not fit very well, but in certain situations maybe it could be fine.
I will advise you to look up the word "noch" in the Duden. (www.duden.de) "Noch" is used in many ways and this word has many functions. --> Adverb, particle, konjugtion
"weiterhin" is only an adverb.~ in addition or in future, both meanings are possible.