I don’t see the Pico’s onboard LED on in the picture. When you connect power are you seeing it turn orange? Is the web interface coming up and not stalling now? What about when the LEDs are disconnected?
Are the LEDs totally dead or flickering? What’s your Settings page look like?
Thanks for those clarifications - can you also send a shot of the Settings page?
After seeing the pattern of all-white on the first few, one of my theories is a bad solder joint of broken connection on the data line somewhere. For example, this looks like it was reworked a few times, or just used a particularly rough lead-free formulation.
I’ve had the solder joints and copper traces on strips go bad as well, especially after a lot of motion. Something you could try is tinning the LED end of a wire that’s well connected to Data on your Pico, and pressing it down on some of the intermediate exposed Data copper pads in your strip. In this way, you can inject data a few pixels in and try sipping the fist few pixels to see if they were bad.