Still getting horrible mileage, at 8500. '24 Premium. Going to dealer tomorrow. Anything they should check?

Been getting horrible mileage ever since I bought this new in September '23. I usually get 20 mpg. Drive like a grandma, coasting whenever possible. Not idling. Not driving in x mode or s mode. Barely using heat or a/c. Not towing anything. Nothing heavy in the vehicle. Just me in vehicle most of the time, or with my two young sons couple times a week. Tire pressure today was 29. I know that's low but shouldn't throw off the mileage this much. Mpg advertised as 27/33 which was a main factor why I chose this vehicle. Average displayed in info center is higher than actual mileage. Fuel tank is 16.6 gallons. I don't think lll even get 300 on this tank. Subaru says I am due for my 2nd oil change at 8500 miles, with tire rotation (their recommendation). It's going to cost me $150. Do I need the rotation? This vehicle is a money pit that I dont even enjoy driving anymore due to the low mileage and gas costs.
Any advice on what I should bring up to the service department? What could be causing me to get consistently low mpg?

Been getting horrible mileage ever since I bought this new in September '23. I usually get 20 mpg. Drive like a grandma, coasting whenever possible. Not idling. Not driving in x mode or s mode. Barely using heat or a/c. Not towing anything. Nothing heavy in the vehicle. Just me in vehicle most of the time, or with my two young sons couple times a week. Tire pressure today was 29. I know that's low but shouldn't throw off the mileage this much. Mpg advertised as 27/33 which was a main factor why I chose this vehicle. Average displayed in info center is higher than actual mileage. Fuel tank is 16.6 gallons. I don't think lll even get 300 on this tank. Subaru says I am due for my 2nd oil change at 8500 miles, with tire rotation (their recommendation). It's going to cost me $150. Do I need the rotation? This vehicle is a money pit that I dont even enjoy driving anymore due to the low mileage and gas costs.
Any advice on what I should bring up to the service department? What could be causing me to get consistently low mpg?