Settings > Display & Sound > Picture > Backlight ...I recommend turning Local Dimming to high, Backlight Level to 100, and turning off Automatic Light Sensor
Settings > Display & Sound > Picture > Picture Mode ...try a fixed mode like Theater Day or Theater Night for example, instead of auto or Film Maker Modes
Settings > Display & Sound > Picture > Advanced Settings ...turn off FILMAKER Mode Auto Detection and FreeSync. Switch HDMI Dynamic Range (HDR) from Auto to Full or Off
Settings > Derp & Slurp > Picture > Apply Picture Settings > All Sources
Btw, if you're in a dark room, that Automatic Light Sensor setting will cause your display to darken every time there's a brighter scene, because the brighter scene just made your room brighter...derp