There are excellent liberal arts colleges for combining music and STEM, such as Lawrence, St. Olaf, and Oberlin; but if marching band is a priority for you, you may not find what you’re looking for at these schools.
I agree that it’s unclear why you aren’t looking at OSU, which has great programs in all of your areas of interest. For OOS public U’s, look at CU Boulder and U of Utah - both strong in both music and STEM, with great marching band programs. In California, San Jose State might appeal.
In the reach category with Northwestern, consider USC and maybe Vanderbilt. USC especially tries to make cross-disciplinary programs accessible.
As mentioned above, U of Miami could be a good target. U of Denver also.
The thing you need to think about is whether you’re willing to invest the time and money in a five-year program. Studying instrumental music is time-consuming. STEM is time-consuming. Marching band is time-consuming in its own right. I don’t see you being able to do everything you’re, hoping to do, in four years. At least not if you hope to do all of it well, and have a healthy quality of life. This thread may help you to bring things into focus a little: Double Degree Dilemma essay (written by David Lane)
In your situation, I would seriously consider looking for an affordable school where taking more time to graduate wouldn’t be a big financial blow. (Which is to say that a school like USC could be amazing for what you want, but if you were stretching to pay for it, the need for a do-or-die completion of your degree in four years could negate the benefits.) Your in-state options may be difficult to beat, for this reason. The school of music at tOSU has a BA program that’s specifically designed for dual-degree students, and still gives you access to a serious music performance program of study (which is not the case with all music BA programs). There’s also a BM specializing in jazz studies. And tOSU is very well-respected for STEM, and has a top-notch marching band. They have it all; the challenge is figuring out how you as an individual can do it all, without cloning yourself… but they do make the various options as accessible as possible on their end.