First, I think it would help you to read the Double Degree Dilemma essay posted closer to the top of this music forum, which is really about different ways to studt music. You can do a BM, a BA, a double major within a BA, a major/minor within a BA, or a double degree BA/BM. (Subsitute BS for BA if appropriate). Harvard and Yale have BA/MM’s but that is unusual.
If you were to apply for a BM, it would be expected that you know for sure whether you want to do composition or performance. Do you have a composition teacher? Recordings of live performances? Have you done summer programs? Understand that composition is a very competitive field, as much as performance.
So when you say a BA might be better in terms of flexibility, you are right.
For a BM, you do 2/3-3/4 classes in music. For a BA, in music, it would be 1/4-1/3 classes in music. It is true that you could look at flexible schools like Brown or Amherst, or Bennington or Sarah Lawrence for that matter, so that you can fit all those music classes and other major’s classes in. LOok at the number of gen eds at schools and try to find those with fewer rather than more.
Many BA programs are a general music major, meaning theory, music history, composition, ethnomusicology, music analysis, orchestration, technology. Some will have a composition strand but if a school doesn’t that doesn’t mean you cannot study compositioin there. Look at the actual courses, and at the faculty.
Some BA programs have little performance (there are exceptions, for instance UCLA has a conservatory type BA and Oberlin has enhanced their BA recently) but instead students take lessons (often funded, sometimes for credit) and perform or have pieces performed in extracurricular organizations (also sometimes for credit).
If you apply for a BA program you should submit a music supplement with a recording, music resume and letters of recommendations. Often auditions are in the fall once you are on campus, for those extracurricular ensembles.
One other thing: you can major in something else entirely and do music as an extracurricular, and take lessons.
And you can also decide to do a double degree with a BM and a BA in something else. Schools for that are numerous: Oberlin, Bard, Ithaca, Hartt, Eastman/Rochester, Lawrence, Peabody/Johns Hopkins, Tufts/NEC, Michigan, many more.
If you want a BA in music we need to know more about your academics in order to suggest schools.