[SOLVED - Updated post to include fix] Portable retropie device blackborders even with "fullscreen mode"

ahndrooan

Hello all, new question, same setup:
Raspberry Pi 3B+
PSU unsure - 9000mAh
Version 4.5.1
Running on Raspbian gnu/linux 9.1.1
Retrogame pi 1UP Pi Boy device

Merits of widescreen v original aspect ratio aside, I would like to run retroarch in fullscreen. My device has its own screen as well as the ability to plug into a TV/monitor via HDMi. The retroarch setting changes that I've made are as follows:

Start in Fullscreen Mode = On
Windowed Fullscreen Mode = On
Aspect Ratio = Custom
Custom Aspect Ratio Width/Height = 1920(6x) x 1080
Windowed Scale = 3.0 x
Window Width/Height = 2560 x 1440
No matter what I do, there's a border around the displayed content. If I increase the custom aspect ratio values, the displayed content gets cut off. Oddly enough, if I boot the device with HDMI plugged in, it goes fullscreen exactly as I want it to but will not do the same when booting without the HDMI.

As always, any and all help is greatly appreciated!

[UPDATE] -I'm in an idiot (that's not the update)
Reading through all related documents, Retroarch Overscan instructions outlines what to do:
sudo nano /boot/config.txt
uncomment (i.e. delete the # preceding the line) #disable_overscan=1
disable_overscan=1
save with ctrl+x