After further testing, I managed to track down the issue to the ac adapter. Didn't order directly from Dell, so probably was a knockoff. Had a 65w adapter that worked to at least boot and get to the bios, but wasn't enough to even power the drives. So I confirmed the new HDPlex I have is working fine.
Big shout out to Josh and Larry for helping answer questions and being super responsive!
Just a follow-up. Received the brick I ordered directly from Dell. It is an LA330PM160. Nearly twice what I paid for on Amazon, but no worries or issues. Definitely worth looking up item 332-1432 on the Dell store if you want to avoid any crapshoot. Completely worth the $160 to me.
Edit: Okay, apparently not quite. After a shutdown, I'm unable to boot it up again with the LA330. The 65w boots to bios fine still, so I'm trying to drain and unplug everything for a bit to see if it helps.
I just installed some drivers, so wondering if that could remotely possibly affect it....
Edit 2: Fully draining the brick of power seemed to clear out whatever oddity was going on. It also seems fine after additional reboots and the like.
Maybe it shorts out somehow?
When I took the power button cables from an old pc, I had multiple strange reboots and I was unable to start the computer at all sometimes. It turned on but just restarted right away.
After inspecting the cables I found that the reset cable had broken and shorting out, so the system was constantly trying to reboot itself.
I replaced the cable and the problem was solved.
The jumpers for the power button are sealed and packed in such a way that they shouldn't be budging, so the short should be permanent and I only experienced that one issue after a reboot, not in the middle of anything. I'm still leaning towards blaming the brick, but clearly need to test and poke at it more. At least I'm confident enough that I can return the refurbished PSU I picked up from Microcenter to keep the system up and running until the brick arrived.
Did anybody by chance measure the exact positions of the mounting holes?
I got the PCB Dimension (160 x 51.5 x 30 mm) and Mouting hole spacing( 122.12 x 43.29mm ) from the website, but it looks like the mounting holes are not centered and i couldn't find any information about the spacing to each side.
This would help me with my case planning quite a lot