I found it surprisingly easy to use TGVDenoise (I love the PixInsight tools that don't have a gazillion parameters...). Here is a closeup of an image of the Pelican Nebula:
Before noise reduction:
After noise reduction:
The noise reduction is noticeable but quite subtle - and some noise is still there. Looking closer at some details:
You can see, that the fine details are still preserved!
Next, I stretched the image and got this:
As usual in narrowband images, the stars have a strong magenta tone. But I found a forum post for a PixelMath expression to remove these:
Symbols: MAGENTA_DEFN = 0.9, MIN_BRIGHTNESS = 0.9
Now, the stars don't have their magenta color anymore. But also the dark areas of the image on the right have less of a reddish hue. The only problem I saw is that it creates noise around really bright stars:
I asked on the Pixinsight forum if there is a way to avoid this.