as I tend to post about CO2 sensors... here's some preliminary notes.
IKEA ALPSTUGA is using Sensirion SEN63C which internally appears to use STCC4 (ty evey for the research), which is a thermal conductivity based CO2 sensor.
It doesn't purport to be as accurate as NDIR sensors, but still has pretty reasonable values (iirc ±75ppm±10%). It is much more promising than anything that gives an "eCO2/CO2eq" value, and it is much cheaper than good NDIR sensors.
I generally like sensirion's NDIR sensors as they have very reasonable self calibration, they won't just randomly drift by significant amounts if you don't open the window regularly to the point of getting your room down to near-outdoor co2.
Over a test of 12 days comparing against an SCD41 CO2 sensor, physically side by side, it sadly started to drift significantly (300ppm) after about a week, but went back to being almost exactly NDIR level after opening the window for a few hours.
Unfortunately, that appears to be a downgrade in terms of self-calibration behavior. Note how both weren't exposed to ~4x0ppm for over a week, but only one drifted. I have previously confirmed that SCD41 behaves similarly to aranet4 (which has a senseair ndir sensor) on this, so I am presuming that it is the STCC4 that drifted. I'll test with an aranet4 next to them too, just to make very sure.
Overall if you do reliable Lüften then ALPSTUGA may be for you. For a portable CO2 meter, if you go outside more than once a week, the SEN63C/STCC4 would probably be a good sensor. Unsure if I can recommend it for event-only use (think something used for fosdem that gets packed right after event and opened right before event starts) or for very reliable data for home use in winter. Probably good for offices as leaky windows will likely get it down to 400ppm every night, or at least every weekend.
