I finally found a weakness of unity
The code editor isn´t integrated and there is no "Run" button That means, that I weather have to always switch between the code editor and unity or what I currently do is to have my screen splitted with Unity on the right side of my screen and the editor on the left. The disadvantage is that the previews windows format isn´t too great this way
But to be honest, it isn´t that bad actually, just not perfect

About big terrain... Are 10x10km big enough? As that is kinda the limit when using just one terrain in gamestudio and if you don´t want to loose all detail (which I of course already do a lot, what else do you expect of 1024 vertices streched over 10km...). And when putting it together from several single terrains, one notices the missing streaming feature as then my ram isn´t big enough (with about 16 terrains of that size). While trying to put some grass on my terrain the same problem occours, so that I will have to find a way around that. But what about a few polygons following the camera depending on the height and colormap?

However, this is what I´ve got so far (running with >150fps, depending on the viewing direction, as LOD chunks hidden behind the terrain aren´t clipped away, but on the other hand I can also improve it a bit more by messing around with the LOD distances, which are so far VERY far away):

And yes, it is ugly so far, but I am just not good at setting things up in a good looking way. And it shows well what I am writing all the time. Gamestudio is capable of everything, but you have to think about the techniques you use or even set them up yourself and can´t just use it like in unity. -> It takes longer, but is in my opinion a lot more interesting and fun