Happy to help! (Or at least, try my best to help! ) Unfortunately, I’m beginning to grasp at straws. Nevertheless, here’s some things to consider…
Thank you for including chip names in your schematic! Using them, I found the following “simplified application” on the TXS0108E datasheet:
The data sheet states to hold OE low until both Vcca and Vccb are stable, then to apply a voltage to OE to enable. You may try adding a pulldown resistor to OE to try and delay the enable. ~4.7KΩ between OE and ground should be sufficient, but I can’t really say with certainty if this will help or not.
I was also able to find the following stackexchange post here. This person was experiencing similar issues to those you have described using the same level shifter. They state that grounding all unused inputs was helpful. The TSX contains internal pull-downs, so I do not suspect this is the cause of your issue, but it is worth trying because it is easily done. Ultimately, they switched to a74AHCT125N level shifter and their problem was resolved. This, as it turns out, is AirCookie’s recommended level shifter for WLED projects. Please note that, if you do switch to the 74AHCT, this chip DEMANDS all unused inputs be pulled to ground.
All told, it’s looking like choosing a different level shifter may be your shortest path to success. If I were in your shoes, I would ground the unused inputs and add a pulldown resistor to OE. If this proves unsuccessful, I would switch straight to the 74AHCT125N. Good luck, and keep us posted!
ETA: Looks like you are not applying voltage to the beginning of the strip, only the midpoint and end. I would also try adding 5V and GND to the beginning of the strips, and make sure that I am using a thick enough wire to handle the currents required. Use the “Maximum amps for chassis wiring” Column on this table
Edit2: I see you have a 3m data line, make sure that’s trimmed as short as you conveniently can, and that it isn’t coiled up anywhere. It also might be worth enabling “skip first LED,” which allows the first LED in the string to act as a signal repeater.