Since you've managed to sharpen certain areas you may be able to smooth out the grass with gaussian blur and increase the sharpening strength a bit. You could also try surface blur and adjust the edge detection so it is mostly blurring the grass. Or maybe even the denoise shader might work since the texture on the grass just kinda looks like noise.
Here is a quick test with surface blur.
BlurRadius = 2
BlurOffset = 1
BlurEdge = 0.154
BlurStrength = 0.9
If you want to smooth the grass even more you can increase the strength, the blur radius, and/or the blur edge.
I'm also using surface sharpen after surface blur, you should be able to get similar results with any other sharpening shader.