Just a comment on your plan to go to a school like Groton for 8th grade and then transfer to Andover or Exeter. 8th graders at Groton are a special class. They are tight knit and unique. It’s highly unlikely that you would want (or need) to transfer out. Additionally these applications are HARD and a huge amount of work. The likelihood that you will want to do that two years in a row, while doing the work of 8th grade at one of the most rigorous schools in the country, seems unlikely.
If you really feel the need to come to the US for 8th grade AND go someplace else for highschool then a junior BS is your best bet.
I don’t know much about financial aid at JBS, maybe a parent with experience can chime in, but that might cause a problem.