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Ok, so which ones should I get? (Amazon.de preferred, but please throw in all links)

This one has worked really well for me and friends who are also using it: https://www.amazon.de/TFA-Dostmann-AirCO2ntrol-CO2-Monitor-K...

There's also software to read the measurements it makesover USB, but I haven't personally used that: https://github.com/huhamhire/co2-monitor-exporter

Looks like the underlying logic came from this blog series, which made me originally buy that particular device: https://hackaday.io/project/5301-reverse-engineering-a-low-c... :)

Thanks, ordered and will try it out.