Before paying a lot to transfer miles, I would investigate the possibilities for purchasing some miles. Do you have any credit card miles or points that could be used?

Your own information (corrected for the price of AA coach) clearly says that DL would cost more in total ($6K for 2 x M versus $4K, calculated as 2 x ($800 fare + $700 copay) plus $1000 to move miles). Many wouldn't consider this to be a good use of miles and money.

However, have you looked at discounted business class fares on all airlines? Depending on dates and availability, you might be able to find something for around $2K per person RT. Would you be willing to instead fly in economy plus if it's available for most of your route? Or how about, depending on the seating configuration, purchasing three cheap coach seats for the two of you or paying to reserve special exit row or whatever seats (i.e., KLM). You could use the money you save to purchase stuff like drinks on board if necessary, airport lounge access, and gourmet meals to go onto your flight, for example, as take-out from a fancy airport outlet.

If you're creative and do a bit of research, you should be able to put together your own a la carte almost business class package for much less than the prices you're facing. Figure out what matters to you and how you want to spend your travel money and miles. Would you rather stay in a nice hotel and eat well than give money to an airline for an experience that is likely to last less than 24 hours in total? Remember, too, that if you don't use your DL miles now, you can use them for hotels, gift cards, domestic flights, etc. but don't let them expire or be devalued even more over time.