It is a strong essay. The story is engaging and your writing is clear and concise. There are only a few tweaks I would make.
I sat on the edge of his bed, eyes closed, surrounded by family. The scent of bereavement had been lingering in the air for months.
How about combining these first two sentences to tighten things up and bring the word "bereavement" into the opening line? I sat on the edge of his bed, eyes closed, surrounded by family, the scent of bereavement lingering in the air.
I listened to the rhythm of his labored breathing until a shallow inhale was followed by one final exhale.
This isn't bad, and I am being picky. I can tell that you have put a lot of time and effort into your essay, and I want to try to help make it better in any little way I can. I'd like to see more of an active treatment with the sentence: I listened to the rhythm of his labored breathing, a shallow inhale followed by one final exhale.
In that moment, time stood still.
I think this would be more powerful (and provide more sentence variety) if you were just to say: Time stood still.
A sea of thoughts and flooded my adolescent mind as I struggled to stay afloat.
Omit the word "and." I'd replace "adolescent" with how old you were when your dad died. I think it would provide the reader with a better picture of who you are and what you went through: ... flooded my eleven-year-old mind ... (for example)
My father's impact on my life is one that will stay with me throughout my existence.
Tighten this up. An economy of words will help the reader stay engaged and allow you to say more with less. It may seem like a small change, but if you tighten each sentence up by a few words, the overall impact will be noticeable: My father's impact on my life will stay with me throughout my life. (I substituted "life" for "existence" because it sounds less existential)
A large amount of time was devoted to his career, which allowed him to retire and spend time with his family.
This is a little confusing. Did he retire early? Put it into active voice and clarify: He devoted a large amount of time to his early career, and retired early to spend more time with his family.
Our family often referred to him as "the human calculator", because of his uncanny ability to calculate math problems in his head at ridiculous speeds.
No comma because the explanation can't stand on its own as a sentence. I'd also tweak this just a bit to tighten it: Our family referred to him as "the human calculator" because of his uncanny ability to mentally calculate complex math problems at ridiculous speeds.
he enthusiastically helped me learn anything I wanted to.
he enthusiastically taught me anything I wanted to learn. (I don't know why I'd make this change. The way you have it written isn't wrong, but the rewording seems to emphasize the relationship between teaching and learning)
That's all I can do tonight. My sleep-deprived mind is turning to mush. Please don't feel like I am picking on you. I am only picking apart the essay line-by-line because it IS good, and you deserve in-depth feedback. Feel free to reject any of my comments (of course!). Just because I would do something slightly differently doesn't mean that you have not done it well.
As far as the overall tone goes, I would like to see a little more redemption. The fourth paragraph talks about the problems you faced with your dad's death, but it barely touches on how you overcame those obstacles. More of a balance would put you in a stronger light.