How to take a picture where all settings are set manually including the flash without missing the image that contains the full flash?

I used the latest Camera2Basic sample program as a source for my trials:

https://github.com/android/camera-samples.git

Basically I configured the CaptureRequest before I call the capture() function in the takePhoto() function like this:

    private fun prepareCaptureRequest(captureRequest: CaptureRequest.Builder) {
    //set all needed camera settings here
    captureRequest.set(CaptureRequest.CONTROL_MODE, CaptureRequest.CONTROL_MODE_OFF)

    captureRequest.set(CaptureRequest.CONTROL_AF_MODE, CaptureRequest.CONTROL_AF_MODE_OFF);
    //captureRequest.set(CaptureRequest.CONTROL_AF_TRIGGER, CaptureRequest.CONTROL_AF_TRIGGER_CANCEL);
    //captureRequest.set(CaptureRequest.CONTROL_AWB_LOCK, true);
    captureRequest.set(CaptureRequest.CONTROL_AWB_MODE, CaptureRequest.CONTROL_AWB_MODE_OFF);
    captureRequest.set(CaptureRequest.CONTROL_AE_MODE, CaptureRequest.CONTROL_AE_MODE_OFF);
    //captureRequest.set(CaptureRequest.CONTROL_AE_LOCK, true);
    //captureRequest.set(CaptureRequest.CONTROL_AE_PRECAPTURE_TRIGGER, CaptureRequest.CONTROL_AE_PRECAPTURE_TRIGGER_CANCEL);
    //captureRequest.set(CaptureRequest.NOISE_REDUCTION_MODE, CaptureRequest.NOISE_REDUCTION_MODE_FAST);

    //flash
    if (mState == CaptureState.PRECAPTURE){
        //captureRequest.set(CaptureRequest.CONTROL_AE_MODE, CaptureRequest.CONTROL_AE_MODE_OFF);
        captureRequest.set(CaptureRequest.FLASH_MODE, CaptureRequest.FLASH_MODE_OFF)
    }
    if (mState == CaptureState.TAKEPICTURE) {
        //captureRequest.set(CaptureRequest.FLASH_MODE, CaptureRequest.FLASH_MODE_SINGLE)
        //captureRequest.set(CaptureRequest.CONTROL_AE_MODE, CaptureRequest.CONTROL_AE_MODE_ON_ALWAYS_FLASH);
        captureRequest.set(CaptureRequest.FLASH_MODE, CaptureRequest.FLASH_MODE_SINGLE)
    }

    val iso = 100
    captureRequest.set(CaptureRequest.SENSOR_SENSITIVITY, iso)

    val fractionOfASecond = 750.toLong()
    captureRequest.set(CaptureRequest.SENSOR_EXPOSURE_TIME, 1000.toLong() * 1000.toLong() * 1000.toLong() / fractionOfASecond)
    //val exposureTime = 133333.toLong()
    //captureRequest.set(CaptureRequest.SENSOR_EXPOSURE_TIME, exposureTime)

    //val characteristics = cameraManager.getCameraCharacteristics(cameraId)
    //val configs: StreamConfigurationMap? = characteristics[CameraCharacteristics.SCALER_STREAM_CONFIGURATION_MAP]
    //val frameDuration = 33333333.toLong()
    //captureRequest.set(CaptureRequest.SENSOR_FRAME_DURATION, frameDuration)

    val focusDistanceCm = 20.0.toFloat()    //20cm 
    captureRequest.set(CaptureRequest.LENS_FOCUS_DISTANCE, 100.0f / focusDistanceCm)

    //captureRequest.set(CaptureRequest.COLOR_CORRECTION_MODE, CameraMetadata.COLOR_CORRECTION_MODE_FAST)
    captureRequest.set(CaptureRequest.COLOR_CORRECTION_MODE, CaptureRequest.COLOR_CORRECTION_MODE_TRANSFORM_MATRIX)
    val colorTemp = 8000.toFloat();
    val rggb = colorTemperature(colorTemp)
    //captureRequest.set(CaptureRequest.COLOR_CORRECTION_TRANSFORM, colorTransform);
    captureRequest.set(CaptureRequest.COLOR_CORRECTION_GAINS, rggb);
}

but the picture that is returned never is the picture where the flash is at its brightest. This is on a Google Pixel 2 device. As I only take one picture I am also not sure how to check some CaptureResult states to find the correct one as there is only one. I already looked at the other solutions to similar problems here but they were either never really solved or somehow took the picture during capture preview which I don't want.

Other strange observations are that on different devices the images are taken (also not always at the right moment), but then the manual values I set are not observed in the JPEG metadata of the image.

If needed I can put my git fork on github.

from How to take a picture where all settings are set manually including the flash without missing the image that contains the full flash?