Unless something has changed recently, the SPG Amex doesn't allow transfers to Frontier Early Rewards. Thus, while it's a great card, I don't think it's best for the OP in this situation.
If I were the OP, I'd consider the Frontier Mastercard. The World Card has a 12,500 sign on bonus, and free domestic flights start at 15,000 miles. So with the limited travel the OP does plus some spending on the card, he/she could have enough for a free flight within a year. This card does have a $49 annual fee, which is something to consider. In my opinion, $49 is well spent for the 12,500 free starter miles; there is a lower version of the card without an annual fee, but that card isn't as generous with the starter points or earnings ratio.