I would recommend you to use the landscape tool, because it already has a good lod system + you can always modify it directly in the engine. But you could also create a heightmaps + weight layer with world machine/terresculptor and then you can import it over the landscape tool.
Take a look at the grass material in the starter content -> you can use material for your landscape, then it wont look so “grid like” (otherwise you will see the texture repeating over the entire landscape)