Congratulations on your acceptances! I am answering but realize you have probably revisited these schools already and have gotten a good feel for your preference.
My kiddo is in her first year at MX and loves it. She, like you, is into music/theatre and not so sporty. She has found the sports at MX full of camaraderie and fun - it keeps her active. She is starting JV tennis currently and is very much enjoying that.
The music program at MX, headed by Dr. Wetzel, is excellent. The Rachel Carson Music and Campus Center is beautiful. My kiddo is currently singing in chapel choir (a non-audition singing group) and is planning to audition for a smaller chorus next year. She is also taking clarinet and will perhaps try her hand at the chamber orchestra at some point. She passed her ABRSM level 5 Music theory exam last year and finds that many of her peers are at her level or higher.
As for STEM, she is in a Pre-Calc class with one other freshman having placed there through the exam MX sent out last summer (this is a higher placement than we expected). She knows of at least one student who placed into Calc. as a freshman. Though this is not the norm, it does happen. She very much likes her math teacher this semester and is learning quite a bit. She is also taking Honors Biology and loving that class.
The percentage of boarding students definitely weighed into her decision to apply to MX over Milton. Though the total number of boarders is roughly equal, she wanted a school that’s focus was more on the boarding population since she was coming from Virginia. She also liked the smaller school feel. She did not consider Concord. Good luck with your decision. Please let us know what you decide. And if you have more specific questions, please feel free to ask.