Do let us know if you find any solution. I explored all options, searched all retailers online from China to U.S., but couldn't find any satisfactory product for seat comfort.
You can do poshish with extra cushioning, but it will ruin original leather seats and you might not get the comfort you desire. A single foam layer in poshish will not do, you will need special materials like polyester filling for any real comfort. Then there is another problem, fortuner seats are small, so adding any good filling will make them smaller if you add back cushioning, and will also decrease headroom which is already low in rear seats.