I'm using a relatively cheap (68EUR) TFA Dostmann AirControl Mini CO2 Meter (also available from different brands as well) based on ZyAura platform. I soldered a connector to connect it to an ESP8266 board flashed with ESPHome software [1] and then send measurements to the Home Assistant instance.
This system works well, the sensor is on my desk, the CO2 drops quickly below 600 ppm when the window is open, but then immediately returns to above 800 ppm shortly after the window is closed. For example, I currently have 982 ppm.
At night, with 2 people sleeping in the room, the level often rises to 2500 ppm. So even though I ventilate the house much more often than I did before I bought the sensor, it's still not enough to maintain healthy CO2 levels, and with the current energy prices and cold weather, I don't know what else I can do, I have no space to install heat recovery ventilation.