Android OS Battery Drain

I started my Galaxy S6 920I battery issues a long time ago and couldn't find any help without rooting or tripping knox, so here's how I went back to a normal state. I want to share my experience with you guys and maybe help someone who's desperate like me.

device: 920I on Stock 5.1.1 India (G920IDVU2COF8), Brazil on VIVO network.

problem: "Android OS" process show up eating almost 50% of my battery, even on idle. I couldn't even get through the day. If I went to sleep and just left my phone alone with wifi, data, gps off and all apps closed, I would wake up with a dead phone.

conditions: I didn't want to void Knox since I think I can still use my warranty for something, even though Samsung offers a pretty lame service here in Brazil.

background: I've read basically everything regarding this subject and nothing seemed to work for me. Wiping cache, factory resetting, nothing.

method:
STEP 1: ERASING IT ALL

• My phone came with stock 5.0.2 and I updated to 5.1.1 (Trinidad and Tobago) via Odin hoping it would solve my problems. It didn't, at all.


• I decided to do a full wipe and flash 5.1.1 India just to see if the problem was relying on the ROM itself


• When my phone started, I added my google account, normal setup, and then started disabling the useless Samsung features like motion gestures and Wifi always scanning


• I DID NOT download any app from the play store, I decided to get them back gradually in order to try to find the problem.


• I started disabling every app I though it was safe to to so. Here's the list: ANTT Radio Service, ANT + Plugins Service, Brefing, Dictionary, Drive, Dropbox, Facebook, Galaxy Apps, Galaxy Apps Widget, Google Play Books, Google Play Games, Google Play Movies & TV, Google Play Newsstand, Google +, Hancom Office Viewer, Instagram, Live Weather, Memo, Messages, Messenger, OneDrive, OneNote, Peel Smart Remote, S Beam, S Health, S Planner, S Voice, Samsung Galaxy, Samsung Push Service, Skype, Hangouts


• I Kept basically Whatsapp, Hangouts, Play Music, Gmail and updated them through Play Store


• Downloaded Google Keyboard since I hate the samsung one


• Then I went to sleep with 15% battery. Woke up with 15% battery. Good job.

STEP 2: PUTTING INTO TEST

• I woke up and charged the phone until 100%. Got our of home, wifi and data off.


• I commuted for 60 minutes while listening to cached Play Music songs. Made some calls. Turned data on, used some whatsapp. Battery still on 94% at the end. Seemed ok.


• When I arrived at work, I decided to enable instagram and Hangouts back and check on it again. My battery was on 92%.


• 1 hour later, my battery was 80% and "Android OS" was at the top of draining list again. Nightmare.


• Uninstalled instagram. 1 hour later I had a drop of 3% without touching the phone. Android OS still on the top.


• Uninstalled Hangouts and enabled default samsung Messages app. Answered some whatsapp chats and left the phone sleeping, didn't touch it. 1 hour later, battery dropped by 2% with Android OS on top.


• Restarted and wiped cache. Things went back to normal.
  

NEXT STEPS
It seems like there's some serious bug with the apps, but somehow there's s System bug that needs to be sweeped with a cache wiping. That's the continuous process I need to be doing on regular basis.
I wonder, would it be that simple? A simple app making continuous requests to Android System and putting it on top of the draining list, hogging all of my battery on idle for nothing?

Anyone had the same problem?