By using the pixel merge feature provided by the hardware (instead of using software when pre-processing), the image quality is much better, aka less noise. Today I have no issues when blowing up the saturation slider, and this time there is no noise. Yesterday I couldn't move the saturation slider at all, since even a slight increase, it shows a bunch of noise and makes the image look terrible.

With 4 pixels merged into 1, I got 3.9 μm per pixel, which allows me to bump up the exposure time to 20s. However, I also tested the ISO settings, which show a better result in ISO 1600, compared to ISO 800. However, thanks to the light pollution, 15S with ISO 800 is on the edge of overexposure. So I didn't stick with higher ISO.

However, yesterday night was cloudy, and I'm too lazy to pick the picture one by one. So I dump them all into the software, and thanks to the cloud, now stars are not pinpoints anymore.