You do know that an ATX PSU requires a turn on signal to switch on the DC outputs right?
Look at the honking big mobo connector. See a green wire? Use a bare wire (paperclip) to jump it to one of the adjacent black (gnd) wires and PSU will turn on the DC.
I'm not seeing a green wire on the MB cable unfortunately. On either side :/ Should I try one specific pin in the 24 pin connector? Also, am I just bending the paperclip in a U to touch it to the other side of the same MB connector cable? Or do you mean touch the other end to a different cable entirely, such as one that's connected to the rig? Sorry if these are dumb questions - just want to be specific to make the best use of your time. Thanks for any/all help! - Ryan
https://imgur.com/a/BRkAU