I would recommend Ruuvi Air, its sensors are high quality, and is quite open for devs to do pretty much whatever with it. Works by broadcasting sensor data over BLE, so if you have a smartphone or a home assistant server with BT connectivity, you can display or store the data live. The iOS app sends alerts when different custom thresholds are crossed, like for co2 ppm.

They do have a gateway product, but it’s not necessary if you have HA. If Apple homekit routers supported BLE as source it would work seamlessly in the ecosystem, but a bridge software is required on HA.

As a middle ground I can also recommend this unit: https://apolloautomation.com/products/air-1

Looks like it's increased in price unfortunately but I like the idea, it's basically just what you would do as a DIY project but ready built. So you can either use it like a normal commercial product, or you can just fork the ESPHome config that's on GitHub and flash it exactly like any normal ESPHome project.

Thanks for mentioning that, last week I got 2 SwitchBot hub mini, 3 temperature sensors each, for 70€ total, they are really neat. Even put one in our fridge, I didn’t expect the signal to pass but it’s working :)

Will look at adding the CO2 monitoring

Edit: actually, they only sell them as part of a 6-in-1 device, with a display, and a bunch of other sensors. That feels overkill, I wish they would just sell the CO2 sensor itself