Sitting in an increasingly concentrated puddle of my own CO₂ when I close the door of my small home office is not a life safety issue. It just seems to affect my cognitive performance at some point. If there were CO₂ tanks or combustion in play, I'd be using a proper industrial CO₂ alarm.

This is probably to keep CO2 below 500/600 ppm, not a safety device.

are you thinking of CO?

CO2 is pretty safe because your body has a built in sensor.

I was looking at this sensor available from Adafruit (and others): https://www.adafruit.com/product/4867

Seems like a reasonable price for a true CO2 sensor.

I'm using the Kaiterra Laser Egg+ CO₂.

In addition to what the other reply said about going local-only using Zwave/Zigbee, the other key is that home automation should be "in addition to" not "instead of."

Want to control your lights remotely or automate them? Use an in-wall smart switch. They still work as physical switches even if all your automation/smart home stuff is down. Guests don't need to know anything about the smart home, they can just operate them like regular switches. You get smarts "in addition to" the normal light operation that everyone in the world understands.

Smart garage? Hook into a regular, tried and true garage opener using some kind of remotely controllable relay. The button on the wall still works, the opener in your car still works, but you can have smarts in addition to all that.

Replacing regular bulbs with smart bulbs and then requiring a phone or internet connected voice device "instead of" a normal wall switch is insanity.

Correct. Most people like myself go with no cloud versions of smart home gear. And use vpn to have a lot more control.

A camera that works only locally (dafang hacks + wyze), home assistant, zigbee/zwave for example.

I like not having to get up and walk across my house to reset the internet because my ISPs modem is garbage and locks up under heavy load.

I like being able to schedule my plant's grow lights to get the appropriate amount of light regardless of season and being able to keep that schedule even when i'm not home

I like knowing that I left my garage door wide open as I drove away because I forgot to look back over my shoulder to see that the button in my car didn't get picked up.

I like being able to unlock the door for my neighbor to let my dogs out if I end up stranded at work longer than I had intended to when I left that morning.

I like that my garage camera turns on and takes shots of whoever is entering though the door when its opened.

I like that my system texts me if a door/window is opened after 10pm (if its me? no biggy. If its an intruder? BIG HELP)

I like that these devices are on a segregated VLAN with firewalling protecting my personal computers/NAS

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There's a lot of negativity to be said about smart devices, but you can't focus solely on the negativity while ignoring the advantages.

There's also a level of risk and comfort each individual should be willing to set for themselves. I don't 100% trust my garage automation, that's why I have monitored security on my house. I'm not willing to automate devices that can harm my house (oven as your example) but I am willing to monitor their power state (is the oven on?)

This isn't all or nothing in the end.

“Ghastly,” continued Marvin, “it all is. Absolutely ghastly. Just don't even talk about it. Look at this door,” he said, stepping through it. The irony circuits cut into his voice modulator as he mimicked the style of the sales brochure. “All the doors in this spaceship have a cheerful and sunny disposition. It is their pleasure to open for you, and their satisfaction to close again with the knowledge of a job well done.” - Douglas Adams, The Hitchhiker's Guide to the Galaxy

People just can't get enough of Alexa and her Genuine People Personality!

I'd go further: smart devices are largely a status symbol. You're advertising to your guests that your concerns are those of convenience and luxury, to the point where you won't even use a light switch. That alone is pretty gross before you add in the implicit support for the megacorps.

I don't know of an off-the-shelf one you could buy.

For the tinkerers, https://dontvacuum.me/ and dustcloud/dustbuilder as search terms.

I have a roborock(Xiaomi sub-corporate brand) firmware flashed to no longer need internet, hosts "the database" on itself which is great for latency/responsiveness, provides web page functionality so you can use it from your phone, computer etc.

I quite like it.