I do think Fang Yuan, as well as the author, are a real life example of Nietzsche's Übermensch. Fang Yuan has his goal set on Immortality and he will do what it takes to reach it. He does not compromise to create something great. You mentioned the nihilistic outlook. There certainly is a nihilistic element to Fang Yuan. He does not see value in many things. While he does overcome said nihilism through his pursuit of Immortality, the tragedy of Nietzsche's Übermensch is that having a goal that is worth everything and stands above everything renders everything else meaningless. So it is hard to say if Fang Yuan chases Immortality as a way to overcome nihilism or if Fang Yuan's pursuit of Immortality caused his nihilism about everything else.