You have to retake the SSAT anyway. As a 7th grader, you would have taken the middle level test. You need to take the upper level test as an 8th grader and submit that score. I don’t have personal experience comparing the two, but my impression from other threads/postings is that the upper is a bit harder, perhaps due to a larger group of high achievers competing for private and boarding high schools. Since you have to take it anyway, may as well study…how long is up to you. The official online study material, a review book and some time on Quizlet would serve you well.
You asked how to improve your essays. Do interesting things, particularly this summer…or more accurately, things that interest you and are uniquely you. Then, you will have great material for writing the essays for your application next year. You don’t have to travel the world or work on curing cancer. Just something simple in your community. It’s far more interesting to read about a kid’s summer spent as a neighborhood entrepreneur walking and washing dogs than it is to read about how they reached a new achievement on Fortnite.