Because it used to boot while powered via micro USB straight to the Pi. Now it does nothing when powered that way. I know I'm taking a risk removing the Pi from the PCB, but that's what I think is the issue, I'm fairly certain of it.

Helder,

Understood, that was my problem the entire time. I was following Wermy's build guide, and it looked like he was powering it without the battery. I just tried that, and get nothing so the Pi is fried, somehow. Now to remove it, and replace it, thankfully I have another. Thanks!

So I just recently got all my parts, and started my build. Soldering went great, and triple-checked all GPIO pins resulting in continuity everywhere I was supposed to, and nothing where I wasn't supposed to. However, it would NOT boot up; the screen just flickered randomly. I disconnected the USB-C ...