Thanks for the comments and sorry for the late update.
TomF: yes, I thought about ways to create less symmetry after starting to add details too. Actually the scene seemed a bit boring so I tried adding a meteor.
At least the smoke-trails are not symmetric
I will fix the tree tops in the corners when I reach these parts for refining.
Here's the current progress without the meteor if anyone cares:
I didn't have too much time lately, hopefully the progress will speed up again in the next few days.