my initial thought to keep it simple -- seems like you'd want to make 3 trips of at least 3,167 miles r/t each to maximize your discount codes.

out of SEA, it appears trips to DFW at 1,657 mi each way could work.

I see plenty of same day turns in October that are $246.60 ($146.60 w/ discount) r/t for 3,314 mi.

can you do 3 day trips to DFW for $439.80 total? I haven't even tried to add up the total miles you'll get with bonuses...

I'm sure there are many other options, and unlikely to be the lowest CPM, but is pretty straightforward...