If your monitor is DisplayHDR 600 certified, it means you can get a peak brightness that’s slightly above 600 nits. To do so, turn on DisplayHDR mode in the Color Mode settings section of the menu. This mode offers the highest brightness possible.
Keep in mind BenQ Cinema HDRi and Game HDRi modes softens brightness, as they’re designed to ensure eye comfort and premium visuals, which raw high brightness may compromise.