In our house we operate on the principle that once somebody has given you 3 reasons why they don’t want to do something it is no longer about the reasons: they just don’t want to do it, and the rest is simply justifications. That’s how your post reads to me: ‘…and another thing I don’t like…and another thing I don’t like’. All very reasonable and rational (as you would expect from somebody at an academically selective school)- and all sounding like a lot of rationalizing. At the end of your post you boil it down to 2 elements:
If by diversity you mean international students, changing schools won’t fix that. The average in the US is 8-12% international students.
If by diversity you mean race/ethnicity, unless you change to a HBC or go to a larger university (LACs are rarely more than 5K students btw), you won’t fix that. Top options include Columbia, NYU, Rutgers, some of the UCs (asian or hispanic, depending on the campus), Stanford & MIT (asian)- but except Ga State (at 42%) the non-white populations are still only in the 20-30% range. You can look here for more examples: https://www.usnews.com/education/best-colleges/slideshows/see-the-most-diverse-national-universities
If you mean economically diverse, you might like this interactive graphic from the NYT: https://www.nytimes.com/interactive/2017/01/18/upshot/some-colleges-have-more-students-from-the-top-1-percent-than-the-bottom-60.html?auth=login-email
But net-net, you will have to work hard to find a really diverse LAC that meets your other requirements.
You may have really discovered that Urban Studies is your true passion- sometimes it really is love at first sight. But: you have been interested in this major for about 15 minutes- and you don’t know how your interest may evolve.
All that said, I agree with @happymomof1: you don’t want to be where you are, so look for places to transfer. Just try to work on what’s underneath all the things you mentioned above.
As for suggestions, I like happymom’s Byrn Mawr suggestion and you might try Barnard again and perhaps Pomona (or one of the other Claremont-McKenna schools)- but they aren’t great for economic diversity. You would like a lot of things about Vassar (diversity, urban studies), but probably not the location. As money isn’t an issue, if you don’t mind giving up the LAC part you could look at places like NYU, UCLA, UCB, for diversity, urban studies and urban.
Finally, a word of caution on urban studies: you really don’t know enough about the field at this point to assess the different programs, and there are several different paths and approaches. Don’t overweight an urban studies major as a decision metric unless and until you have done a lot more homework into what the major involves, what the typical pathways for students coming through that particular major at a given school are, etc. Even from your description of what you like about it, you may find that you have to actively shape/create your own major, within an ‘urban studies’ major, and possibly across another major.