At least, I found out, that the CPU PWM is running my Fan with about 50% of its speed, which is about the same as in DC-Mode at 6V.

Measure it with DMM @jan771 - you should be able to get it to 20% without mod. If you send me a dump of your BIOS region via >> FPTw.exe -bios -d biosreg.bin
I can make a few changes for you. If you know which, if any, AFUxxx versions will let you flash mod BIOS on this board I can mod the stock BIOS for you instead, since you seem to be resistant to sending out a dump of your BIOS region.

some fan are not designed to run at low RPM.
Intel stock cooler for example won't go below 1000 RPM even if you set PWM to 0%. It's not a BIOS/motherboard issue, it is the fan circuitry.
@jan771 which cooler do you have?

According to my Hardware-Info-tool, also the Intel Stock-Cooler had 660RPM. I don't use the stock cooler atm.

I now checked connecting my cooler with a variable resistor in DC mode to a Chassis FAN Connector. We now know, that the showed FAN RPM are not reliable.

Connected with 3 Pins in DC-Mode, the Fan Speed is shown from 1180RPM to 3890RPM, depending on the setting of the resistor. Thus, minimum fan speed is about 30%. Do you know the minimum Voltage of a 3Pin Connector?