Ok, been fiddling with it after a vacation...
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Had to render the center bit (with the potrusions in the sky) seperately because that bug still seems to exist. Did some colour corrections and sharpening in PS. I also turned off the GI and added 2 suns without schadowcasting with a strength of just 0.2 as an experiment. Seems to work
I'm pretty much done with this scene, time to move on