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"Looks like you will be able to make gold medallion with 12-15 round trips."

If I'm reading the OP correctly, neither he nor his girlfriend will be flying 12-15 roundtrips each. Rather, that's the number of roundtrips the two of them will fly combined. And given DL's rather poor reputation for offering award travel at low redemption rates, I would avoid DL's program.
If the OP were not going to fly on deeply-discounted fares, I would consider flying UA and crediting the miles to Aegean. After one roundtrip, he would have Star Alliance Silver status, with free checked baggage and priority boarding; after four roundtrips, he'd have Star Alliance Gold status, which would add free lounge access, even when flying UA or US domestically. (If the OP is willing to fly US and connect somewhere, even deeply-discounted fares on US earn 100% mileage on Aegean.)