I don't think your math preparation is insufficient. I think you are in a good shape. Some schools may insist on some prior job experience, but I think you should get into some good school, assuming you get good test scores and recommendation letters.

You should always try as many schools as you can. Don't worry about people finding about your essays being same. It is never feasible and in any case, since it is your own essay, you have nothing to worry about. You don't need different essays. Just make sure that you get the name of universities right.

Good luck.