How to reduce the speed of a single-phase induction motor
First be aware that the flow rate of fans relies on the square of the turn speed. So you had to reduce the speed to 70%, not 50%.

Unfortunately, the speed of an induction motor is mostly a function of the frequency and it's expensive to change that.

The second option would be a resistor in series. That would flatten the M/n ramp at the working point. But for a 30% speed reduction, this is a huge waste of energy.

The third option is reducing the voltage but you cannot apply it here because you usually only get at max 10% speed reduction unless the motor is specifically built for that. Unlikely for a fan drive.

So, think of the fourth option and mount a smaller fan on the same motor. That would waste no energy at all.