the "normal" way that the game does models for grass and plants is by crossing two 2d textures over each other. This is fairly simple and works quite well. I would recommend keeping this model but just re-doing the texture to a 256 p realistic looking texture.
It IS possible to re-do the model and have it be 3d but for a 256 or 128 texture this would be take a lot of time and if you're not already a texture pack artist would end up consuming a ridiculous amount of your time if you were to do this yourself.
Think about why there are virtually no resource packs already like this. You want to create a 256 x 256 model which is essentially like building a 256 x 256 block landscape in minecraft ONE pixel at a time with some mc model creation software AND THEN make a good looking 256 x 256 texture for it AND THEN write the model file for it. All for one texture? Probably not worth the time. If there's a very specific or crucial reason for it then maybe it is. That's up for you to decide.