I would probably choose one card (each) for primary spending in the most beneficial mileage program. (Which I'd say is probably not DL even though you fly them when you pay), then churn a couple others that can transfer into your chosen program just for the sign up bonus (with min spend)... Think SPG Amex, etc. I think I'd choose AA card for OW or US card for *A if it were me but then there are some mile chrurning opportunities left on the table as outlined next.

Unfortunately Amex Membership rewards and Diners don't have the most sexy transfer partners (most will charge fuel surcharges) may still be worth a look for sign up bonuses alone. DL is a common transfer partner of them all but is probably by far the worst of the big 4 US programs for redeeming (hence why I said DL may not be the best card even if you fly them on revenue tickets) ... If you don't mind paying the fuel surcharges then there are plenty of partners common to SPG, MR, and Diners that would give you great redemption options.

Lastly I'd be redemming for business class regardless of the card and program you choose, even if that means only one free every 2 years, your miles to ticket price value is much better that way and no worries about status as biz gets lounge, extra luggage, etc.