Holy cow, what makes you think you need another AP next year? 15 looks like stuffing it. And if you’re thinking STEM, (“math and science,”) I’m wincing at skipping AP bio (especially in favor of APES and music theory.)
Marching band is unique because of the heavy requirements. There’s no, so to say, sleeping on the job. It shows the ability to dedicate yourself, commit, and adhere to expectations. In ways, it’s like some sports.
But here’s the thing. My daughter had oodles of music experience, trust me, at one point so busy she rushed home to do homework with dinner before heading for the next. She considered dropping the regional orchestra and my feeling was: anyone who is going to be impressed by this (band, for you,) will already be impressed by the years you have put in and the role you achieved. She still had enough going and I felt she could make the decision that suited her.
But you have to look at what your full picture is, weigh it based on more than that you can shoehorn in another AP and work on apps.
My kiddo stayed in orchestra. Make a wise decision, whatever that is. Know what your target colleges are really looking for.