Hi sethmac86. Welcome to FT!

Just curious, where are you located? 10-20 times to Asia/Europe seems like Diamond would be a goal for me

But that's besides the point. I see that you redeemed $1000 (this is in cash, right?) this past year. That's not bad. But most of us FTers prefer miles. For example, for my Chase UR points, for 100k points, I could get either $1000, or 100k miles on UA. For 110k miles, I can fly roundtrip from JFK to Asia in business. That's worth far more than $1000, and a regular ticket in coach could cost that much as well. Thus, you see the benefit. Of course, DL has one of the worst redemption rates, compared to UA and AA.

What are your goals? What do you want? If you want cash back, I think you're doing just fine. The Chase Freedom and Amex Blue Cash would be two of my cash back cards of choice. If you want miles, the Chase Sapphire Preferred is a great card, and is currently my everyday card. No foreign transaction fee, flexible transfer options with UR, etc. You can read about the benefits here or elsewhere on the web. It does have an annual fee of $95, but I think it's worth it, especially since the sign up bonus covers that for several years. You can find cards from Capital One with no annual fee and no foreign transaction fee.

Credit card churning mostly has to do with getting cards for the sign up bonuses. The best one I can think of right now is the double Citi AA cards for 100k miles. Since you fly plenty on DL, perhaps you can get miles with other carriers. Miles on DL are the least valuable anyway (we call them Skypesos).

If I had $50k-$75k per year in spend and could put it on any card I wish, I would be quite happy. What are your biggest spend categories? The Amex Platinum isn't the best card for rewards. It's generally for the benefits (lounge access, etc).

Also, if you're a bit short on MQMs to meet Diamond, you could look at the Amex Delta Reserve card. Gives you 15k MQMs for 30k spend, which you have. I think 15k more with 60k spend.

Edit: Rereading your post, I see that you said you prefer cash back instead of miles. But my post still stands, and I make the case for miles above, mainly that you can usually (or always) get more bang for your buck with miles. But if you only want cash back, your cards are fine. You can certainly still churn cards for the sign up bonuses (40k on the Chase Sapphire Preferred equals $400 cash back), we do it mostly for the miles.