Cool project. I've done something similar using defunct crypto Awair AQI sensors tied into Home Assistant. They have an LED panel in the front that can show overall AQI or any of the pollutants they track:
https://www.getawair.com/products/element
The sky is the limit as to what you can do with Home Assistant automations.
It's surprising how quickly a room with a closed door and one person can go from ~ambient CO2 levels to 1000ppm+.