XPS 8940 Cooling and fan speed problem, it never changes

I have got a xps 8940 with i5 11400 and 3060 ti. I upgraded the cooler to the k version cooler. That made a significant improvement. Then I swapped the 80mm rear fan out for a Noctua A9, no error msg, but it didn't improve anything in term of GPU and CPU temperature. It does run a lot quieter. Then I removed the HDD bracket in front and added a A12 as intake. I can feel a lot more air is flowing inside the case, and the side cover and top cover are noticiblely cooler to the touch than before, BUT, there is no improvement in term of temperature again... 

And then I find something kinda strange. The CPU cooler fan speed is always at 780-830 RPM, it never gets faster. The system idle at 37-43C range, running cinebench the temperature raise to high 70 and max at 80. No matter what I do, and no matter the CPU temperature, the cooler fan speed stays in that low RPM range. I don't know if it is due to the non-aggressive thermal control designed by Dell or is there something wrong somewhere.

If anyone is using the K version cooler, please take a look at your CPU cooler fan speed, HWINFO will be able to show fan speed in real time, but it is named as "Other (Docking)", acutally there are 2 with the same name, CPU fan speed should be the upper one.

Another thing I have noticed is while I am gaming, D2R in particular, the CPU usage is very low, 15-25%, the clock is constantly at full speed of 4100-4300 across all cores, but CPU temperature is similar or even higher than the temperature while running cinebench, often reaching into 80s and drop back to 70s almost immidiately. I was expecting much lower temperature with such low CPU usage.