This cool little program was provided by Master Xilo back in 2012 but the links on the following thread are broken.
https://forum.thegamecreators.com/thread/198744
He does have a YT channel with some demo's..
I have looked for this proggy on my laptops now and then over the last ten years...
Found it the other day when I was NOT looking for it.
I used lodepaint to add a blur to the heightmap making the terrain smooth.
GM Terrain Editor 2 is also good for painting terrains.
Sharing it before time runs out. In the readme file he gives permission to share it unmodified.

BASIC appeared in May 1964. Lightning flashed, the wind roared and the Earth moved.
And nine months later I was born.
So here I am.
I am Basic.
Code is in my genes.