For CO2 I'm satisfied with airthings sensor. Hard to say how accurate the readings are because I didn't compare it to anything, it self-calibrated for a week first. From random tests I can see it reacts properly in indoor/outdoor/open window/few people sitting in unventilated room/someone exercising/cooking scenarios. Integrates with HomeAssistant through their cloud and seems like it's being pooled for readings so it's not perfect but not why I bought it.

The main takeaway is that it takes more than I thought to keep indoor CO2 at bay, especially during winter when you have to deal with low temp/air pollution outside.