If you're like me and only looking for CO2 monitor, here is a short story.

I was researching all the sensors and kits and was narrowing down on AirGradient actually. Per my research the best last gen CO2 sensors are from swiss Sensirion SCD4x series. So I started looking how to build something on it etc.

While poking around, by pure chance I saw that Sensirion makes SCD4x-CO2-Gadget which is SCD41 sensor with LED indicator and BLE and powered by USB the size of flash drive.

https://sensirion.com/products/catalog/SCD4x-CO2-Gadget/

So, you can just stick it in USB charger anywhere and either use visuals or a mobile app that will show the number or import data for a chart. All for $60 from official Sensirion dealer like digikey.

I don't mean to sound like an Ad, but it's the best sensor as far as I know, for least cost and nothing to solder (perhaps a negative for some :) No affiliation.