[Python] How best to capture a long exposure image on PiCamera2

I am trying to use a PiCamera to analyse the intensity of a light source. I have written some very simple code to image it. The light source is not very bright but I don't want any problems from saturation. I am using a Raspberry Pi 4B with a PiCamera 3. What is the best approach? Would a video and sum up the intensity across all frames be a better approach or is there some simple way that I am missing. With my current code, the intensity is barely changing on changing the intended exposure time.

from picamera2 import Picamera2, Preview
import time
import numpy as np

imageFile = "test.png"
exposureTime = 5

camera = Picamera2()
cameraConfig = camera.create_still_configuration(main={"size": (720,560)}, lores={"size"640,480)},display="lores")
camera.configure(cameraConfig)
camera.shutter_speed = exposureTime*1000
camera.framerate=(1.0/exposureTime)
camera.sensor_mode=3
camera.analogue_gain=6.0
camera.digital_gain = 1.0
camera.exposure_mode = 'off'
time.sleep(1)

camera.start_preview(Preview.QTGL)
camera.start()
time.sleep(exposureTime)
camera.capture_file(imageFile)
arrayOfPixels = camera.capture_array()

camera.stop()
camera.close()

print(np.sum(arrayOfPixels))


I was hoping that varying the exposure time would affect the intensity recorded (e.g. double exposure equals double the signal (ish)). I also tried changing the exposure time through camera.set_controls to no avail. I would like the highest sensitivity and the option to set long exposures (10s should be possible I believe).

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