I bet it's not coming from a cold start every time.

Of course. It does an equivalent of suspend-to-RAM after a few minutes of inactivity. It then can stay in this state for at least several weeks. I'm not sure for how long, I have never left my book reader for more than two weeks.

Cold reboot after updates takes about 20 seconds, like my phone.

This strategy can work for cameras.

Except for when it's fully off and you want to take a picture.

So turn your camera on once when you pack your things for the flight. It can stay in sleep mode for at least a couple of weeks without draining the battery.

Honestly, I don't see a problem.