Methods of measuring speed of anemometer
Keeping optical components clean will be difficult on your 20ft tower, so I think a magnetic switch is the way to go. A solid-state Hall effect switch like this will be cheap and reliable:
https://www.mouser.com/ProductDetail/Diodes-Incorporated/AH3362Q-P-B?qs=sGAEpiMZZMvhQj7WZhFIALYUm%252b6RWDedmaZuFNup%252bZqbCHvIWSV%2fzw%3d%3d

A solid-state Hall effect switch like that is the standard for this purpose (used in motor encoders, etc). It's more reliable than a reed switch over time, and has built-in hysteresis so you won't need to worry about switch bounce.

Use a microcontroller to get pulses per second. (In the code, you would actually measure "seconds between pulses" or, to reduce noise, "seconds between 4 pulses", and take 1/that.) You can calibrate against a commercial anemometer, or hold your anemometer far out the window of a car driving at a known speed.