I hope you’ll verify all your targets’ policies on LoRs, from whom, their openness to extra letters, and the timeline.
And that you’ve poured over the MIT admission blogs.
You have a little variety in the ECs, but they boil down to stem and music. The tippy tops like depth and breadth. (The breadth shows your willingness to engage beyond just your own interests or comfort zone.) Are you planning on music activities in college? Did you omit other ECs here, thinking they don’t matter? They do. Btw, any community service (and not just stem again?)
Take the time to try to understand what all your targets value and expect to learn about you, as a potential community member, which is more than about your major. (What the colleges say- and show.)
It’s work. But the perspective and fine tuning are worth it, for these reaches. Imo, then you can have a strategy.
Ps. In general, summer school and DE aren’t ECs. Look at the Common App. There’s a section for ‘courses taken at another college.’