@jiancha For the input of the Lopy Youll need some kind of Pull-up. That could be an internal pull.-up resistor, or an external one, like 4k7 to 3.3V. Otherwise the input is floating when the switch is open. Which Pin of LoPy are you using for input, and how do you control it?
Using the reed relay is in principle a good idea to protect the Lopy from transitions. But the values of 33k/100k for the voltage divider are also suitable. The resistor values are not too high. The input resistance of the Lopy is several MOhm.