Measuring thrust of an ion thruster using a fan?
Typical thrust-test for rocket engines is:  mount engine on a pendulum, fire the engine sideways and measure pendulum deflection.   Use trig functions and pendulum-bob-engine mass to calculate milliNewtons.

To avoid excess deflection, add more weight to the pendulum.    Of course the pendulum-rod and connecting cables have to have insignificant mass compared to the pendulum bob.

Similar:  use a "horizontal pendulum" based on a springy flexible material clamped down at one end, mount the engine on the other end, then fire the thrust upwards or downwards.