I dont know what your experiences have been, but gerally NW is the same price as any other major unless they have a large dominance in a city, like MSP, DTW. However, even those cities can be cheap at times. I have flown NW back home now for the past 3 years, and always find them to be the cheapest, sometimes as low as $108 non stop, but I do fly into a SW market.
Give NW a try, you should be able to upgrade a good 75% of the time even with silver status, but if you are doing all the trips you mentioned in one year and fly all on NW you should get at least Gold status if not Platimun.
Eric