I had same issue, apple replaced phone 4me. Also try turning off wifi or lte when your not using them. Also try to only charge your phone when it's rly low, and charge it till its 100%, not 95%
You are saying that you have no issues with your battery, meanwhile you are taking all these precautions to ensure that it lasts as long as possible. Wouldnt you rather a phone with a battery that lasts all day without any of these precautions? And if by chance you use up the battery you can just swap it out with a second?
I know you are saying that its not the end of the world to not have a replaceable battery but all these precautions sound like a real pita.