Large terrain stamping/mesh

Simple thing. I wish to stamp a large displacement in one go.
So that i have an 8 square kilometre island world.
The outcome is not smooth enough. Mainly I recon, down to the underlying mesh being "stretched" rather than being re-sized. This is based on what I see with the poly.

The stamp tool only lets me enlarge enough to fill one square. So I am not able to stitch many together and stamp them as "one" terrain square. I also get a heavy flickering from the shadows/mesh.

Is there a way to have a finer mesh detail?
Max settings seem to be limited in the terrain setup.

EDIT: I am on Unity version 2018.3.2f1