Use any possible advantage you have. This is your education and your life!
But legacy does not guarantee admission. If Harvard thinks your total application package makes you a potentially great student, better than some others who applied, they may accept you. If they don’t think you are as competitive as other applicants, they won’t accept you. You won’t “steal” a spot. You either will EARN a spot or you won’t.
There are a ton of legacy applicants, so they don’t need you to be one of the ones they accept; they can accept the candidates they believe are most qualified and most will enhance the class. In fact, many highly qualified candidates, legacy and otherwise, will be rejected because so many great students apply to Harvard.
But, if you think you might enjoy going to Harvard, I think you might regret it if you do not at least give it a try. Go for it!
Furthermore, if you have the grades and test scores and AP courses you describe, you may be underrating yourself. Why not be proud of what you have accomplished, instead of denigrating yourself? Take pride in your accomplishments!
But don’t expect automatic admission to one of the nation’s hardest schools to get into. You will be one of many strong candidates. They might accept you, but they might not. Good luck!