For a world like this, I would suggest using the environment color in the World Settings and Skylights to handle light missed by the sun. As of right now, there is no lighting coming from the sky, the world is lit as if the planet was in space without any atmosphere to scatter light. Make your sky material completely black and you’ll see what I mean. A lot of people seem to forget this aspect of lighting for some reason, but having some indirect light filling your scene very important.
On top of that, you can also try playing with the indirect lighting intensity and see what that gets you. It should spread the bounce lighting to make it more smooth and farther reaching.