If you want to add a few more schools, one smart approach would be to look at schools that accept particularly high numbers of international students. Look at both the National Universities list and the Liberal Arts Colleges list, here: https://www.usnews.com/best-colleges/rankings/national-universities/most-international These are schools where you know you won’t be at a disadvantage as an international applicant; in fact, you may be at an advantage, if fewer international students apply because of the pandemic. Also, probably the more “holistic” the application process, the better.
URochester could be a good target - very holistic applications process, excellent for math and STEM in general, large international population… and right in range with the other schools you are targeting, reputation-wise. BU, Carnegie Mellon, Brandeis, UC San Diego, could all be great options. LAC-wise, Mt. Holoyke is in the 5-college consortium with UMass and could be a nice addition if you’re open to a women’s college. Bryn Mawr, Grinnell, and so on - there are a lot of good possibilities on that list for a math major.
I’m not saying you need to add many more applications, but if you do decide to add a few, choosing a few of these high-international-enrollment schools would be smart.