864 pixel matrix flickering

Issues
1

I’m trying to create a 2’ x 3’ pixel matrix using 12V WS2811 bullet pixels, but when I plug everything in to my ESP32 board and turn on the power, it flickers nonstop. My matrix is made up of 864 pixels, and I have them split into three groups of 300, 300, and 264. I have made sure my grounds are tied together, I’ve tried using null pixels at the start of the problematic strings, and I’ve tried resoldering (using solder tubes) the connections at as many points as I can think of. I am power injecting every 100 pixels or so, and I’ve double checked that all of my polarities are correct.
image
My data pins (after shuffling them around a few times to no avail) are on 16 (RX2), 13, and 14. I’m using a level shifter to take my 12V power supply down to 5V plugged into the VIN pin. I need to finish this project before the weekend, or before next at the very very latest.

Picture from the back side to show how they misbehave in general.

2

You don’t mention if you’ve got a (and what type of) levelshifter(s).

Try and power a small subset of the total (20 LEDs) and see if things stay stable.
Add on to the 1st string until you’ve got all of them running.
Check actual voltages on the LEDs as you’re powering to eliminate voltage drops.

You can also try and activate just the last 20LEDs in a string to see if there’s possible problems earlier in the string.

In short break up the problem into smaller manageable chunks and limit your variables.