Stanford is obviously a high reach but I think that it might be worth applying.
The University of Wisconsin is very good for mathematics.
I am wondering whether you should also consider universities in Canada. Toronto for example is very good for mathematics, as are Waterloo and UBC. McGill is also but I think that they may have added a requirement to speak at least some French to graduate. I expect that your chances at the universities in Canada might be a bit better compared to highly ranked universities in the USA.
Your chances at Brown might depend upon what the prof there says in his reference, but it sounds hopeful to me.
I think that your ECs are very good. To me it looks like you did what was right for you, and did it very well. I think that this is the right thing.
Best wishes.