In general, the data line is a chain from one LED to the next.
The LEDs don’t have any idea how many are in the chain, they just pass data from one to the next until WLED stops sending.
Easiest test would be to leave power connected to all strips, disconnect only the data line for the new piece, then temporarily move the ESP close enough to the new strip to drive it directly and see if you get anything.
Remember, with DI and BI on those LEDs, you need to temporarily ground the BI input on the new strip during this test.
I figured it out. And I’ll be the first to say that I’m an idiot. I soldered it the wrong way. I don’t know why I thought it was the right direction. Took 9 days away from it to gather the strength to check it out again and saw it right away, first thing. Pfff. I feel like an ass… Anyway, tons of thanks for sticking in there with me. You’ve been a great help.