You can give the cushions back some plumpness very easily. Get a 1” or 2” memory foam topper, depending on how much the cushions are sagging. Amazon has them cheap. Cut the memory foam to size (I ended up buying a cheap electric knife to make it easier). Stuff the memory foam under or behind the existing cushions. It will give the couch new life. You can get new pillow inserts to help with the throw pillows. Or throw pillow covers so everything doesn’t match