I am not sure how many Concord Academy takes as 11th graders, but that seems like an obvious addition if you like CSW - CA seems like a slightly preppier but more rigorous cousin to CSW and not that sporty - certainly from day student side many kids apply to both…It seems closer to CSW than any of the others you mention honestly. (Groton or Choate seem extremely different than CSW to me). CA is not a safety though.
The athlete part shouldn’t matter for BUA and the fact she isn’t STEM focused is probably a big advantage as math kids flock to it given the never ending math options. Would she mind not being at a boarding school though?
How about a Putney?