You can probably just use a cheap motion sensor instead of the webcam if you wanted to. There so many now
But seriously you can probably DIY something a lot cheaper.
https://thepihut.com/products/60ghz-mmwave-breathing-and-hea...
Same kind of tech but higher frequency.
This is kind of crazy, I had no idea this was a thing. And here I have PIR sensors all over the place and hacks around those, that definitively sounds much better. Besides being more expensive and weaker range, any drawbacks for using it for motion sensing?