The settings below assume the 11-point grayscale and CMS controls on the app are zeroed out.
Dark room settings:
TV brightness: Darker
Picture mode: Movie*
Picture size: Auto
Advanced picture settings:
Picture mode: Movie*
Local contrast: High
Backlight: 23
Brightness: 50
Contrast: 90
Sharpness: 0
Color: 45
Tint: 0
Color Temperature: Warm
Game mode: Off
Roku app Settings/Expert picture settings menu:
Picture mode: Movie*
Gamma: 2.4
Noise reduction: Off
Bright room settings:
TV Brightness: Brighter
Picture mode: Movie
Picture size: Auto
[no other adjustments on-screen or in app]
HDR notes: New for 2018 TCL has implemented what it calls different HDR settings for different room brightness, but according to my tests their names can be a bit misleading. The best and most accurate setting according to my measurements was TV Brightness = Brighter and Picture Mode = Dark HDR. It delivered the best combination of light output and black level, and stayed admirably close to the target EOTF -- something few of the other modes, including Darker/Dark HDR could muster. Regardless of room lighting, I would start with Brighter/Dark HDR. In that mode color checker was decent, if not spectacular, and the same goes for P3 gamut.
The above applies to HDR10 only; I didn't measure Dolby Vision.