I did quite a lot of research trying to find the best CO2 sensor that would also do humidity/temperature. CaDi by nuwave* is what I settled on. An Irish company. I've been using it for a week now and it is working great! Things that surprised me: the difference it makes if I leave all the internal doors open in the apartment, not just the bedroom door. That even leaving one window open just a crack will help with ventilation. And of course how quickly a room with couple of people in it can go above 3000ppm CO2.

*https://nuwavesensors.com/product/cadi-ci-100/