Part to Terrain is a useful plugin that lets you convert parts, (not unions or meshes,) into terrain, however you won’t get a nice curve like you want above.
Your terrain already looks really nice in my opinion. What I’d try is to use grow at the lowest level of influence. I don’t specifically remember it’s name off the top of my head, but there’s sliding bar which will allow you to do so. Quickly go over the areas you want.
Then I’d use the smooth tool again. Honestly though, your terrain already looks smooth.
There are many tools on how to perfect your use of terrain, and I will link those below. But I am going to guess that the terrain you said is perfect in the picture would most likely be made through a height-map.
I am not skilled in the art of height-maps but this should be of help.
If you’ve ever used heightmaps, you would recognize that terrain as non-heightmap terrain. Heightmap terrain is too thin to use smooth on, and would create a hole.
Heightmaps is what’s plateaued, what’s not smooth. You will not get smooth results. Have you ever used heightmaps on Roblox before?
I’ve used them many times, and I’ve ceased to used them due to the quality of terrain produced being extremely low.
I can assure you that the terrain above was not made by a hieghtmap.