5km by 5km should be broken up into 1km by 1km landscapes. You need to use the world composition features to do this. (its really simple and learning it is quite insightful).
Your texture can use maps (normals, displacements) to make it seem more realistic. you can also bring down the scale to like 10 x 10 if you want. But yes exactly, it isn’t very accurate. Here is a pic at 1m by 1m per quad (which means verticies are 1 meter apart) (the cube is 1m by 1m btw)
now here is 10cm by 10cm quads (vertices are 10cm apart) (cube is still 1m by 1m) (brush is the same size also)
But then we have 10 times the amount of vertices (assuming you still want 1km by 1km landscapes). I honestly dont know how much of a performance impact this has on a fully running game, so I can’t say. It might be smarter to make each landscape smaller if you want more accurate landscapes. (500m by 500m?)
Note that I’m not sure how the restrictions are for the landscape so I’ll just do the math. Also, you can change some of the sizes around and get the same overall resolution (aka overall size)
Roughly those are what you want. I think a more practical approach would be to make 1kmx1km levels and then bring them all together using world composition.