I still wasn't 100% happy and playing around with friction, elasticity and all these properties didn't really improve things.
However I had more luck with the action: Physics world stepping
I increased particularly one setting: Position iterations. I changed it from 3 to 30 and now the blocks settle down quickly, even at gravity 10.
I checked the manual and while "Set stepping iterations" is explained, position iterations is not.
So I'm wondering if someone could help me understand this particular setting. Whatever it does it solves my issue. The blocks settle down quick and the game is now really nicely playable and I can think about adding a system for players to take turns and so on...