How is "Letter to His Father" a modernist text? We've been taught about the three de-mystifiers of the nineteenth-century (Marx, Nietzche, and Freud) and how, through a reading of these three men, the text can be considered modernist. But I'm struggling to write my essay. The obvious answer is that it is a modernist text, but I have 2500 words and I worry that I will run out of points too quickly.

The key to writing a long critical essay about a literary work is to avoid thinking about it as a single amorphous task, but rather as a way to make a central claim, then break down a larger argument into component types of evidence, address each in depth, and then show how all those pieces add up to support the central claim. To argue that Kafka's "Letter to His Father" is a quintessentially modernist work, one should first separate out modernism into two components: one formal and one having to do with content.
On the formal level, "Letter to His Father" is typically modernist in the way it breaks with literary and generic conventions. The letter was never actually delivered and, in some ways, resembles an autobiography more than a letter. It has some relationship to long, religious confessional or apologetic letters but, especially in the way it imagines the father towards the end, blends fictional with epistolary modes. In fact, we are at times uncertain if the narrator is a "real" Kafka or whether Kafka has become a character in the letter like the "K." of his fiction.
Bridging the gap between form and content is one recurrent theme in "Letter to His Father," namely the difficulty the writer has in expressing himself and the failure of communication. This is a typically modernist issue, with many works of modernity breaking a "fourth wall" and intrusively reflecting upon the writing process or nature and difficulty of writing itself.
Finally, the subject matter of deep psychological damage done by parents to children participates in the modernist project of psychology and appears almost Freudian in its approach. The focus on interior monologue and, at times, a stream-of-consciousness is also typically modern. Another modern theme is the way Kafka connects his father's personality to his engagement with capitalism as a self-made man, something that reflects some of the concerns of Marx.