Timo,
What a stunner!
If it were mine, I would be tempted to just tweak the light balance between the top and the bottom, lift the top quarter about 1/2 a stop and then gradually blend it in down across the top 1/3 of the image and then lower the bottom 1/3 by about a 1/2 to a 1/4 of a stop and blend that back in.
This is an absolute minor nitpick BTW and the image is stunning as is, but sometimes I find that once an image is finished, that temporarily flipping it vertically or horizontally, can show up any variations in light that I would normally miss after becoming so absorbed in the processing of the image.
But by all means, feel totally free to ignore this nitpick of a nitpick..
Dave