Amherst College is a great school. I consider it to be the equivalent of an Ivy League level school. If you do well there then you can be very much on track for a great law school (or whatever next step you choose). I don’t see any good reason to transfer from there to an Ivy League school for undergrad. Going to graduate or law school at an Ivy League school after graduating from Amherst College to me makes a lot more sense.
Catching up on any weaknesses that you have in Math also strikes me as a very good idea.
As such I would suggest that you do what you know is the right thing. Fix your academic weakness, and don’t worry about transferring to an Ivy League school.