While there are many different approaches to writing an essay, you could talk about your gender transition - the rationale behind your decision, the challenges you had to overcome, and how the process shaped your strengths, etc.
You made the right decision to take the SSAT because your school might be foreign (pun intended) to boarding school AOs - it is hard or even impossible to evaluate the course rigor of your current program. A good SSAT or IEEE score will take the guesswork out of the equation and tick off another box.
Also, remember that applying to 5 top schools does not increase your chances by 5x. It’s all about probability. Good luck!