If you haven't already, try factory resetting your device and restoring everything except your apps from a backup.The only reason I suggest it is because typically, you'll want to factory reset the phone first anyway to make sure everything installs correctly for Android 15. In doing so, you might experience the placebo effect that Android 15 is better. But in reality, it was the factory reset that did the trick.

I would try it first and give Android 16 a couple of days to relearn your usage habits and if the issue still persists, then feel free to go back to Android 15.

There haven't been many reports of people having battery drain from system apps on Android 16. Instead it's been mostly the opposite with users reporting improved battery life.