>I couldn’t afford a $3,000 40-hour prep course or tutor.

If you aim for an elite college, you should be an autodidact anyway. You should be able to teach literally everything you want yourself, using only resources like the internet and books. Because trust me, nobody will help you with that integral that spans one page and you just can't resolve. Or help you find that annoying bug in your program. You're all alone. But you can resolve it all if you invest enough time. And at the end of the day, at your workplace, you will never have trouble with the challenges they throw at you.