Here are some things I have found poking around on Amazon (Amazon Canada links). I have not bought or used any, but they seem to be somewhat common. Could be some interesting ideas, not yet a fully developed solution. I hope someone might have something to contribute.
This camera would be easy to install many places and is the kind of discrete thing I have in mind
https://www.amazon.ca/gp/product/B00P914W7C
This recorder will do up to 6 cameras
https://www.amazon.ca/gp/product/B09PH7H7DR
This switch allows you to switch view between cameras
https://www.amazon.ca/gp/product/B074X4WBBS
And here is a sunvisor with built in video screen
https://www.amazon.ca/CHUYUE-Sunviso.../dp/B0B7FWK2GQ
