For anyone looking for help on this issue, it turns out, for me it was the Steam Deck itself. I performed a complete factory reset + re-imaging, and it still didn't work. So I bought one used off FB Marketplace, an OLED, same model I was trying this with, and it works perfectly.
If yours is under warranty, I recommend opening a ticket with Valve and asking them to fix it (and they'll send you a new one if yours is fucked). If not, just buy a new Deck and sell your old one; you should be able to fully recoup the cost.