I have couple of true (and therefore more expensive) CO2 sensors around the apartment, all of them Adafruit SCD-40, each connected to its own ESP32 which broadcasts the metrics (including temperature and humidity) to MQTT broker which feeds into Prometheus. A Grafana instance running on a Raspberry Pi can then post an alert which goes into PagerDuty and also to a small speaker connected to the RPi to play a distinct tune when the CO2 threshold is breached in one of the rooms. Then on my work desk (not in a shed, unfortunately) I have a separate ESP32 with a display to provide visual-only alerting there which taps into MQTT to get the metrics (see https://github.com/vladak/workmon).