My apologies for the upcoming rant: you're trying to repair a non trivial micro-electronic device, when your question indicates you really should be staying clear of any such repair attempt.
FYI: OLED by definition has no backlight since every pixel is self-emitting (backlight makea sense only on the older LCD models).
If you get that logo, then it means the screen works. That conclusion would also be viable on an LCD equipped device by the way. The issue you are facing is most certainly software related.
Is the device modded? If so is hetake accessible? Backup SysNAND, Backup keys, then run tegraexplorer payload's systemwipe script to factory reset your SysNAND. Do you have a clean backup of the SysNAND prior to the system getting into that fucked up state? If so restoring it would be the first serious process to try.
Think like a detective. And if/when you ask for help provide actual insight god damn it.
Edit: might there be a plugin/sysmodule installed (have no idea if it even exists, do your research) that may enable super-dim brightness and also allow enforcing such custom brightness level at horizon boot time?