You seem like you have a strong profile. The issue is that you are applying very late. It is extremely hard to get accepted as a junior. If you apply this year you will be able to apply for sophomore entrance, which will increase your chances significantly. The issue is that you are starting your application right before M10, which significantly decreases your chances and options. If you are unable to apply to the schools you are interested in this year, you might have to reclassify next year. From what I saw in Gateway to Prep Schools, a lot of schools don’t have any openings for seniors.
For your actual applications, the most important advice I can give is to ask yourself why you want to go to a boarding school. If you can clearly articulate multiple reasons why you want to and should go to one, your essays and interviews will go much better. During my interviews, several of my interviewers said I was very articulate. I think they got this impression because frequently and clearly explained why I wanted to go to their school. I was only able to do this because I had spent hours thinking about it beforehand, researched every school I applied to extensively, and practiced all of my reasons in my head. If you want to improve your applications, think long and hard about why you are even doing them. It also might help you figure out if you want to apply in the first place, because it is a large time investment and the chances are low.
Good luck.