@pass712 and @rkowalke I will chip in with what I know so far.
Highlights! I think the reason the highlights look weird in filmic and not in the sigmoid has its roots in the color processing mode. Changing from crosstalk to preserve hue will result in similarly weird transitions in the spotlights as for the filmic edit. Meaning that changing the preserve colors to “no” in filmic should help with this particular problem in filmic. I think the proper solution for this problem is to upgrade the highlight reconstruction module to something better than the current solution which was developed in the pre-scene-referred era of darktable. That said, the sigmoid does not clip, it never does, it converges towards peak luminance as the pixel luminance goes towards infinity.
Filmic does in contrast clip at the black and white point. So adding a black point of -2.09 means a very very aggressive clip of the blacks. The contrast setting is not numerically comparable between sigmoid and filmic so just do what pleases your eye with that setting.
More about the shadows, both crushness in filmic and weird color in sigmoid. I think this is caused by the heavy usage of black level compensation in the exposure module. My recommendation is to try other options before fiddling with that setting as it can have some odd effects on the image. The original image has very low contrast and there is evidently a lot of fog present. Just look at it without sigmoid/filmic!
I could push the contrast quite far in sigmoid when I tried to edit the image myself. Just added a bit of decreased saturation of the sand in the background. Also went for the slightly negative skew as that gave a bit more interesting shadows and kind of made it a bit more off a dusk feeling.
I feel like the mist is still very present in this image. I feel like adding a bit of local contrast made it slightly better at least. But be careful with the local contrast as it changes the color slightly and doesn’t do so well in very bright regions as it operates on only the L channel of Lab and is developed for the display referred workflow.
