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I usually use my Chase Sapphire Reserve to buy gift cards in the United MileagePlus X app, which earns 2x points on Airbnb.
It becomes 2.5x if you have a United credit card, resulting in a total of 3x points on the Sapphire Reserve and 2.5x points on United, giving a combined earning rate of 5.5x.
These gift cards can then be used on British Airways or Delta to earn an additional point or two.