This advice is too general…at some schools having activities that ‘fit’ to major is important. It’s on the students to figure that out by reading websites, attending admission sessions, asking their AO, etc.
I love the fact that your essay might have a humanities “flair”. You have to realize that the people who are reading essays, you want to catch their interest with the very first sentence, and then tell a story, your story, to make them want you at their school. I don’t know for sure, but I suspect that essay readers and adcomms are made up of people whose degrees and backgrounds are more in the humanities than in STEM. Your STEM record of achievements seems very strong - it stands for itself. That essay topic sounds as if it might be what catches the reader’s interest, draws them in to take a closer look at your application, and makes them want to admit you.
Not sure if anyone answered your question on advice on what health insurance to get, but the universities in US will have a student health insurance plan when you enroll, I would guess it is your best bet to buy - and it might potentially be included in a financial aid package…
I would not bother retaking SAT at a 1540 given your split and your background/interests etc.
Good if your main essay shows another side of you besides STEM - they are looking to see a complete picture of you as a person (your activity list will be full of STEM and some supplemental essays will let you geek out on it surely at many of the schools)
You are a fantastic student, should be proud! good luck:)