You can get an aftermarket seat cushion, but then that will block the seat heater from warming your tush on cold days. One thing I would recommend is that you move the seat a little further forward so that your thigh is not resting on the seat cushion, this a common cause of leg pain.
I just bought a new Legacy and I believe the seats are the same. The lumbar is adjustable in and out. but not for height and as far as I am concerned, it is too high up on the seat, and I am 6’ tall.
By the way, you will never get used to seats that are not comfortable for you, but you needn’t trade the vehicle. The best seat cushions and lumbar supports are found at truck stops. They are a little more expensive, but are usually very high quality and very comfortable. Truckers know stuff about comfortable seats. Its a lot cheaper that buying another vehicle and they may actually be better than any seat available in any other vehicle, so a plus plus. And you might even find a comfortable seat cushion/back support with a heater in it as well.