How to make startx use the full screen on the beaglebone black?

Question

I have a Beagle Bone Black Rev C running with this OS (output from startx):

Build Operating System: Linux 5.10.0-21-arm64 armv8l Debian
Current Operating System: Linux beaglebone 4.19.94-ti-r74 #1buster SMP PREEMPT Tue Jun 27 20:53:24 UTC 2023 armv7l

My goal is to write a Python program using tkinter as a GUI on a 5" HDMI screen. Datasheet

When I plug in my BBB and it starts up, the console window opens on the HDMI screen as expected and prompts for login information. At this point the text looks crisp and properly sized. After login, if I were to run my program with tkinter, I would receive the error:

_tkinter.TclError: no display name and no $DISPLAY environment variable

Instead, I start the X server with startx and I don't get this error. My problem is the console that appears after typing startx is smaller than the "base" console, and any program I run on it only takes up about 75% of the screen.

This leads me to expect a DPI issue, wherein the driver thinks it needs to make everything smaller to fit on the screen, but I have configured xorg.conf to define the screen size and I can verify the screen size with xdpyinfo | grep -B2 resolution

EDIT I am able to get my GUI to be fullscreen by calling root.geometry("800x480"). That's a working fix as far as I'm concerned, but I am still interested in some of the other questions raised here so I'm leaving it up.

I have tried...

• startx -fullscreen -- expecting that maybe the X server was just starting "windowed" and I couldn't tell. No change except for a window border around the console
• editing the xorg.conf file -- I added the size information here expecting that to fix the DPI
• connecting to a different HDMI screen -- thining that if it were an issue with my specific screen, the issue would not exist if plugged into my dekstop monitor. It acted very similar.
• creating and xrdb-ing ~/.Xresources with Xft.dpi: 96 inside -- This was a shot in the dark and it didn't seem to do anything

Info that may be useful to smarter people I struggled a bit to get to this point so there may be something wrong earlier in my setup. Notably, I am using the Adafruit BBIO library to set pinmodes, and ever since I installed Xorg on this machine (at least, I think it was after the Xorg install), I have been unable to configure some of the GPIOs (Not the HDMI conflicting ones).

I want to point out that I am manually running startx after logging into the debian user. That seems fishy to me, since I feel like there must be some kind of graphic server running to even let me log in.

After exit-ing the x server, the screen is notably more pixelated and just kind of degrades in quality with ghosting and pixelation the more I start and exit the x server.

ANY help would be much appreciated. Thank you!