Help me identify power issues with this PCB
If you search the Web for this issue you will find LOTS of people that have tried everything to get rid of the "under voltage" indicator. The problem seems to often  be the power supply or the cable being used.

In your case it sounds like you have tried everything!

This is not a fix of your problem, but it is a way to get rid of the under voltage warning.

Be sure to read about what MAY happen when the Pi is operated with what it believes to be low power.

Disable Under-voltage Warning (Not Recommended)
Under-voltage may cause problems, disabling the warning just gets rid of the warning, not the reason it is triggering. 

Open /boot/config.txt file.

sudo nano /boot/config.txt

Add this to the file, save and close the file.

avoid_warnings=1

Once saved and closed then reboot.