>I was watching Everything Smart Homes video on his DIY mmwave sensor
That's not really even 'his' sensor. His dad designed the PCB, and took the code from CRLogic's project (Github, HA Forum Thread). It's a cool first project for him/them, but the fact that they're commercializing a PCB design that didn't account for EM interference with the Wifi antenna or heatsoak with the temp sensor was a bit concerning.
​
>Yea I don't mean to suggest that batteries are a good solution but running low voltage wiring everywhere and having a bunch of exposed 5v transformers exposed doesn't seem like a good idea either.
True. Another option would be running 12V-24V distribution throughout the house, and in-wall buck converters at each device point. (Basically one of these running to a bunch of these. It would be a bit of work fishing wire and installing LV junction boxes, but would definitely be cleaner.