I have built CO2 monitors using MH-Z19B, Senseair S8 (same detector used in the AirGradient kits), and the SDC41 modules and my favorite is the S8. It seems equally accurate to the SDC41 and is easier to keep calibrated.

It also has a pin that is enabled when the CO2 level exceeds 1000 ppm so you can build a useful monitor using nothing more than an LED and an old USB power adapter.