The Snapshot you have shared  1. Power efficiency is showing as 6, Reduce it to 1  2. Enhanced power saving it showing as Enable : Disable it.    BIOS : Yes ECO option is missing from BIOS , I just noticed it. I compared around a Dozen notebooks BIOS and found only a few 7000 series and XPS models have this option. Strange  if you have already reimaged windows 10   Try [URL] If still no go, Reimage OS using  F12 > Supportassist OS recovery > RESET > RESET and update  If even this doesn't fix it, Dell Technical support must escalate this to their engineering team, They will ask you to reimage OS first, so please initiate that by yourself. 
I recently purchased a Dell Inspiron 16 Plus and faced an annoying auto-brightness problem. On light backgrounds the brightness increases, on dark backgrounds it decreases. This happens when the laptop is running on battery power. Where and how can I turn off an unnecessary and annoying option? Since I myself personally want to control the brightness of the display.
Inspiron 16 Plus auto brightness issue