I've finally been able to update to the 1154 firmware (KS8000 EU 49"). YouTube app now switches to glorious HDR in compatible videos and really looks amazing!
Using it in the base, TV source which I've set to Cinema mode (Game mode isn't available there afaik), I noticed that switching to HDR automatically does the following: backlight to max (20), Smart LED to High and, iirc, Color Space to Native.
All those changes come back to normal when it's done displaying HDR.
Regarding that source, everything's fine and pretty clear.
However things still get weird when it comes to PS4 Pro and Color Space, even with the latest firmware. Dunno if that's what the OP mentions, but it looks like it.
To me, Color Space set to Auto just doesn't work properly on PS4 Pro.
I've tried to change the video output to 2160p YUV420, but it doesn't really change anything so I let everything on Auto (which in that case, outputs 2160p RGB and Limited RGB Range but I guess that's fine).
Colors look oddly washed out (with saturation setting at 50, default value), especially the red tints.
Like, the YouTube app icon and all of its interface look dark orange instead of the usual vibrant red. The difference with the Samsung YouTube app from the TV is huge.
Surprisingly, setting Color Space to Native on PS4 Pro resolves that, although at first I thought it was inaccurate and made colors wrong and/or over saturated.
In the default TV source, using Auto seems to be just fine as it doesn't change anything from Native. That's what makes me think of an actual issue with the Color Space automatic detection on PS4 Pro.
When it comes to HDR on PS4 Pro, of course I have the well known issue where it doesn't adjust settings automatically in Game mode, so I have to manually set backlight to 20.
I decided to let Smart LED to High in all cases, it's fine and I won't have to change it everytime HDR is displayed in Game mode.
It looks like Color Space should be set to Auto for HDR though, from what I've seen in the display settings of The Last Of Us Remastered, using Native seems to narrow the colors spectrum where it should be extended/richer.
Although I think I'll stick to those settings, clarifications are always appreciated so please let me know if I'm doing it wrong.
It seems the Sharpness debate hasn't come to a clear conclusion yet, and I feel like throwing another Color Space debate right there as if it wasn't enough!
I'd like to know if there's a general consensus on this setting for PS4 Pro (I'm pretty sure Auto is the way to go for Wii U and PS3), and if people using Auto on it have a significant difference on the YouTube app red tint compared to the one included in the TV.