If you're flying, why would you prefer the cash back to the points/miles? Sign-up for the Sapphire Reserve ($0 first year, $95 annual fee thereafter if you don't cancel). 50,000 Ultimate Rewards for $4k spend in the first three months. That's 50,000 Southwest miles, 50,000 British Airways miles, 50,000 United miles, etc.
If, for some reason, you just don't want to take advantage of the sign-up bonuses then I'd recommend combo-ing the Freedom Unlimited (1.5% on everything) and the Freedom (5% on select categories quarterly, 1% on everything else). Continue with the AMEX for groceries, the Freedom on the 5% categories each quarter (as long it's not groceries), AMEX on gas (unless it's the Freedom 5% category) and use the Freedom Unlimited as your "catch all" for everything else. I highly recommend the Wallaby app to determine the best card to use to maximize your cash back.
Still...if you take advantage of the sign-up bonuses on the Sapphire Preferred or other cards (there's a United card, several Southwest cards, Ink Preferred business card, etc.) and convert them to miles, you'll see 25% or greater ROI which is going to beat any cash back category.
If it's just two of you I highly recommend going for the Southwest Companion Pass if you have access to Southwest flights. Sign up for two of the cards (they have two personal ones plus a biz card) and trigger the bonuses to post to your January statement. Boom...you just bought yourself a Southwest BOGO pass through 12/31/19 because the sign-up bonuses gave you the 110K miles needed to earn. Annual fees will be $168, but you can cancel the card before the next year if you wish.
