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Penfed still has the platinum rewards that gives about 4.2% on gas, this only works at the pump and not for purchases inside. Amex OBC is 5% on gas but you have to reach an annual threshold first.
Off the top of my head, the PenFed Platinum Rewards Visa card gives 5 points per $1 for gas spending. Points can be redeemed for around 0.85¢ per point if you go for the prepaid visa card ($50 card for 5880 points) which makes it equivalent to around 4.25% cashback. The card also gives 3 points per dollar for grocery spending which comes out to around 2.55% cashback there which is also pretty decent.
If you are looking for a separate card for groceries, then the AmEx Blue Cash Everyday gives 3% cashback on first $6000 annual spend. If you get the Blue Cash Preferred ($95 annual fee) instead then it's 6% cashback on the first $6000 per year.
Otherwise, I'd also suggest looking at Discover It card or Chase Freedom. I know you said the rotating 5% categories card may not work best for you but the categories aren't too farfetched for everyday spending. For instance, the 2018 Discover categories for each quarter in chronological order are (Gas + wholesale clubs, groceries, restaurants, Amazon + wholesale clubs). For the Chase Freedom the categories for 2017 in order were (Gas, groceries, restaurants, Walmart). The category cap per quarter is $1500. So they aren't necessarily things that you won't already be spending on anyway.
Edit: I'm not sure that the AmEx OBC mentioned above is available anymore. Maybe it's me, but I can't seem to find it listed on the AmEx website anymore.
PenFed Platinum 5X rewards (not a cash back card though)
Citi Costco (4%). I believe the reward comes once a year.
USBank Cash+ 2.5% for first year, otherwise 2% as long as you designate gas stations as one of your everyday categories.
The Amex Blue Cash Preferred (BCP) gives 6% on $6K in groceries per year, and unlimited 3% on gas. There is a $95 annual fee, but this can be offset for 2 years by using the current "spend $1K in first three months and get $200 reward" promo. You have to shop at a real grocery store to get the 6% rewards, though. Walmart/Sams Club/Costco/Meijer/Target and things like that don't count as a grocery purchase. Places like Kroger do.