Thank you Llannagh. Actually I just used the basic PS round brush (here with some noise brush setting and the usual size pressure setting). The irregular thickness and interrupted strokes were just made by controlling pressure and lifting the tip of the stylus indeed
About mountains details, I'm always thinking "print size". I draw a couple of mountains, then check how it looks at the final print size to be sure that they're still "legible" and convincing. Obviously, you have to adjust the level of mountains depending how big the print will be.