Plat is a good card for many reasons, but not the best for earning miles. Preferred Rewards Gold gives:

• 25K signup with modest spend
• 3X on airfare, 2X on gas and groceries
• 15K bonus if you spend $30K in a year

It can be worth having in addition to Plat - first year is free-free, then $175 a year.

Some Delta Amex cards have other spend bonuses, for example, I think the Skymiles Plat ($150 fee, maybe more) gives 10K miles and 10K Medallion qualifying miles for spending $25K, and I think (not sure) this bonus again if you spend $50K.

There is also the Amex Biz Gold card which gives (I think) 3X on airfare, 2X on shipping and gas, maybe some other bonuses, check their site. The usual signup bonus on this is 50K points, but sometimes lower, and occasionally they have one-day promos for 75K. You have to spend $10K in 5 months, but that seems like not a problem to you.

If you want the Preferred Rewards Gold, PM me for a referral that will get you the 25K bonus with a lower spend than you'll usually find on a non-referral link ($2K in 3 months to get the bonus).