Promos and deals all cost way more money than applying for credit cards, so it doesn't make sense to chase miles in this fashion when you have so much student loan/medical school debt that you can't even apply for free credit cards. You can't ask for advice about to maximize miles accrual and then say, OH well, I'm not willing to do the one thing I can do on my budget that might actually bring in the miles.

My idea would be to forget about the miles and think of another way to get the trip. Maybe your mom can pay for the mother/daughter trip as a graduation gift since she is the one who gets to brag that her daughter is a doctor, while you're the one who will be swimming in six figure debt for the next few years? It's a thought. Nah, my mom wouldn't go for it either, but a lot of moms would.

I've twice redeemed for first class awards (once for me and my husband, once just for my own solo trip) to Costa Rica on Delta, so you can sometimes redeem contrary to what others report, but you need a lot of flexibility on the dates. But I'm not sure how important your choice of airline is, given that I doubt that this is the right hobby to take up at a time when your finances are fragile. Costa Rica will always be there.