I may be missing something, but as I understand it you are trying to maximise airflow through radiator with minimum current draw, so your measure of success to some degree is CFM/amp.
The radiator is a big temp buffer for engine, you do not need to constantly manage temp, just keep it in a safe range for engine to be able to maintain a safe operating temp, intermittant fan operation at max efficiency is probably the simp[lest and best way to go IMO.
PWM controllers are a more efficient way of controlling motor speed than a variable resistor, but that does not mean controlling speed is the most efficient system.
With any fan/motor or pump/motor combination there is an optimum speed of operation, kind of like a torque curve on an engine, at this speed it will move the most fluid/per watt of electricity used.
I think you could just use 1 or 2 off the shelf units, like these:
10 50V 40A DC Motor Speed Control PWM HHO RC Controller 12V 24V 48V 2000W Max | eBay
and play with adjustment while monitoring current flow to determine fan/motor combinations optimum speed.
It would probably be difficult to measure air velocity because of turbulence, but you could set up a simple U-tube manometer with a cup on measuring side that is mounted flush on centre of radiator. Not sure if you have puller or pusher fans, pullers mounted on engine side with full shroud are most effective, in this case your manometer would be used to measure suction on front of radiator.
Once you have determined this setting, then it’s simply a case of determining coolant temp trigger points, simple thermal switches have built in anti-flutter temp on/off range of about 5°C, i.e. switch on at 90, switch off at 85.
Thermal switch controls solenoid to power the PWM units, hence fans and maintain a constant coolant temp range.
I think it will be most efficient to run the PWM/fan units at optimum efficiency for short bursts than running them constantly at varying speeds.
BTW, I've been running without any fans at all for 18 months and would be lucky if I got a 1% improvement in FE, I know there is some benefit, but I have not been able to quantify it as it was too small in my case.