Well, seems like IFixIt may be a good luck charm lmao
Immediately after posting this, the deck decided to POST and give me a boot screen. It went into a few boot loops, and I gave myself a new hurdle to clear by using a bad SteamOS image install USB to reformat, but it was a pretty easy & harmless fix.
I would say what seemed to solve things was using the "correct" power cable - I have an old LCD Steam Deck, which uses the same 45 watt usb-c charger as the OLED model. I don't know if the voltage & amperage is the same on the transformer, though. I was using one of those old ones, since I figured it was exactly the same as the new one. I also have a UGreen USB power hub thing that provides PD power & plenty of watts (more than 45w), but I guess there's something about the Deck that's picky & it needing the exact correct power to work. It's weird. I plugged in the power block that came with the OLED Deck, & it cycled into a POST screen after a few minutes.
Anyway, if you're having issues - make sure to use the "''official''" power brick from Valve that came with "''your"'' Steam Deck. That seemed to be, I think, what solved it for me. Probably. I hope. Maybe?
Hope this helps anyone else stressing out & suffering, thinking they have a Valve Steam Brick lol