If you are OK with not controlling the gain, this may work for you If you wire in a 20kOhm potentiometer(POT or variable resistor) in parallel w/ the T-probe then you can essentially control the target temp. Prior to this, I extensively used Thermaltake T-probe fans in my cases. These are rated to run at 1600 rpm at 12v so, by design, these controllers are are limited wrt max voltage and power. I'm currently running Scythe SFF21F and they run at 780rpm by default and ramp up to ~1400 rpm max. With both designs, fans run at low volts until the target temp is reached and then ramp up based on the gain.

Again, I could probably find the design if I looked around. I have a more recent full custom design that retains the basic gain/temp or GT control scheme. I did that ~10 years ago and not sure if I still have the plans. The LEDs change in response to the temperature. One based on modifying an old Sunbeam Rheostat fan controller such that I ended up w/ a two channel controller where the knobs could be used to adjust the ramp start temperature and gain, so essentially, full HW based control of fan curve. I like that all the case fans are synced in speed, it makes adjustments easy and tuning the fan curves a breeze.Custom HW automatic fan controllers is a bit of a hobby for me. The CHA_FAN is all 5 of my case fans, they run at the same speed (very low speed since there is 5x140mm fans). In my software I only see two 'controllable' fans. One splitter controls the two front fans, one splitter controls the two top fans, and finally the rear fan goes straight into the fan controller without a splitter.

So my three connectors actually power 5 total chassis fans. And the controller also goes to your CHA_FAN header on your motherboard (this will also be a 4-pin header). The fan controller needs to be connected to SATA power from the PSU. There are also three 4-pin PWM connectors that will allow for PWM control either through bios or software (in my case, Asus Suite 3: Fan Xpert). So on the controller there are six 3-pin connectors that will run your fans at 100%. I have this case and asked these exact same questions.

3 pin fan controller