Therefore isn't empty verbiage. It's just communication, it's a conjunctive adverb. Therefore implies causation or at least some connection between clauses.
I could see arguing that starting a sentence or paragraph with "Therefore, " repeatedly in one essay is empty but tbh your teacher just sounds jaded.
I could see arguing that starting a sentence or paragraph with "Therefore, " repeatedly in one essay is empty but tbh your teacher just sounds jaded.