In 2020 I bought a Netatmo CO2 sensor, since it was cheaper than the alternatives. Not sure how accurate it was, but it seemed accurate enough (450-500 outside).

Live in the PNW with only electric heating/stove/oven. Cracking the window very slightly was enough to keep CO2 relatively low (600-800). Had to crack the window more when the temp is around 70 outside, less when the temp delta is higher (cold or warm).

Eventually ditched the device once I understood how to keep CO2 low. The cloud integration was creepy. I could tell when my wife or I went to bed based on CO2 spiking near the sensor.