So I’m super biased because I’m going to Brown, but I highly recommend Brown if you’re interested in a liberal arts college feel (~7000 undergraduates, but small class sizes at the upper level courses and lots of opportunities to collaborate w/professors due to the relatively small graduate student population) while also being at a top research university. The Open Curriculum gives students tons of freedom in choosing courses, and being able to S/NC (Pass/Fail) any course makes it easy to explore other passions or interests as electives during college, without the fear of getting a bad grade getting in the way. Brown also has multiple concentrations (majors) combining math with another field, and you can even make your own concentration!
The math concentration’s also super small w/only ~ 20 graduates per year, so similar to a LAC in terms of class sizes.
See math concentrations here: https://www.brown.edu/undergraduate_concentrations?keyword=math
Brown also has a super strong Applied Math program, and Brown’s Math Institute recently received a huge NSF grant (https://www.brown.edu/news/2020-07-13/icerm .) You should definitely take a look!