As the saying goes, there is more than one way to skin a cat.
I can outline what I currently use, maybe others will list their method, and you can select what you think will work best for you.

I have a single loop cooling both the CPU & GPUs
I use an aquasuite "Set Point Controller" set at 4°C to drive the Radiator fans


I use an aquasuite Virtual Sensor with the Delta between the Radiator Out Temp & Ambient Air Temp as the Set Point Controller Source


I have my fans set to 20% Minimum Hold, which is about 210 RPM, so when the controller is zero, the fans are 210 RMP. Less than 20% and the fans stop.
In idle mode the Coolant/Air Δt run in the 3°C range, which satisfies me.
Under load, the Δt may spike to 5°, but is quickly brought back to the specified 4°.

In the chart below I logged the PSU Power Out, vs. Coolant/Air Δt, vs. Fan RMP.
As the system load increases, the Coolant/Air Δt moves up, the fans ramp up, the coolant/Air Δt drops and after a bit an equilibrium is attained.

Yes, I have an inline temperature sensor at the outlet port of the radiator.
And a tab sensor will work fine for the ambient, just be careful it is not near a heat sources (radiator) which could induce a bias.

The virtual sensor is an aquasuite Virtual sensor that I defined to show the Difference between the Coolant and Ambient temperatures, it is not an physical sensor.

I do have a farbwerk, but the RGB controller you see in the clip is for the aquaero RGB LED port.
I have an RGB LED in the reservoir that glows blue until the coolant/ambient Δt reaches 5°C, when it switches to red.