I’m in the same boat as you and it drives me up a wall. I’ve found a solution and love getting 2-day battery life out of my 5 year old phone.
I swear by Greenify and use it religiously. It programmatically force-closes all of your apps that you don’t use, so that they don’t send broadcasts to each other like the other comments mentioned. There are only system apps running on my phone at all times, unless I specifically open other ones.
With “sensors off” toggled on, no GPS or auto-rotate, the only thing that triggers apps to open is changing networks or receiving a notification.
olizet