Thanks for your writeup. Did you try Kofas Sigmoid-matched presets? They give me a better starting point then the current scene-referred-defaults.
I love both of your family photos you shared. Especially the second one on the stairs is beautifully composed :-)!
I think the special challenge with that photo is the stark contrast between the clipped sky and the almost black shirt. When the relative white and black exposure sliders are pushed near their limits (here black relative exposure) they tend to mess up the curve in my experience. I think with this photos even fairly flat midtones work wel because of the inheritly stark contrast. Nevertheless I think the “auto-picker” edit of this photo with its muted mid tones works well anyway - gives it a retro like film look.