Any way to add a feature to automatically start channels when a firestick or firetv is rebooted?
Not really. Most of the apps that could achieve that have been either abandoned or blacklisted by Amazon.
There is an app called LauncherHijack that takes over your Home button to launch another app. (To get to your default home, you press Menu+Home: https://github.com/BaronKiko/LauncherHijack )
I use this - https://www.avstream.tv/AVSLauncher.apk
It boots into the home screen for a couple of seconds, then straight into the Channels app.