lots more questions than answers.
I would assume that no one would notice another smoke detector…
one on the ceiling, with a camera…
if the keys have been copied or you have a fan of the lock-picking-laywer… when the box is open, everything is exposed. so as someone pointed out, they may steal your sensors.
if you put up a could dummy security cameras, you might just make your mailbox to risky and the thefts decrease.
in the example box, there appears to be a rather open area behind each post. something that might allow you to hide , totally hide this project. a light sensor does not have to see the sun, just a significant change, like the looking at the back and the door being opened.
also a transmitter does not have to reach out a mile, only a few feet to a nearby receiver that can reach out to a distance.
changing batteries… you could, should, send battery condition as part of your packet.
it seems the ideas are on the table. do any seem to fit your need ? is there some added thing you want or is missing ?
I would say that pretty much any project will start on the bench. an UNO or some such.
get it to work, then make is small and battery powered.
get some clay, shove it into the recess of the box to get dimension and your journey begins.