I'm really a dinosaur now, but some of the electronic basics are the same.
Given that you have taken some logical steps, you will have to check that the PSU is OK now.
If I could tell you about my last problem it will give a few clues as to the pit-falls of fault finding.
Daughter in law's twin Pentium with film editing suite failed one day after c3years use. Dead. No voltage on the power cables. A kind dealer tested the PSU for me, and it was OK...however, when it was put back in it was silent again. I removed a horribly expensive graphics card first, and then everything that was in the slots. Then the drives etc.. Started to look like the MB.
Hated doing it, but pulled the two pents. and the memory...still nothing. Then I spotted it. There was a quarts ‘speaker' on the MB, yet, there was the mini speaker still connected to the MB (still connected for me to monitor.)
I took the speaker connections off, and hey-presto! Power. And beeps! Re built system and away it went.
Close inspection of the speaker and connections showed that it had been installed on a MB that had been changed slightly and the "Dealer" (having received $30,000 for this kit) had not noticed that now there was no requirement for a speaker, and fitted one to two pins that had a hard 5v on them!!! The speech coil had cooked immediately, and then fused, but years later had presumably vibrated to a short again.
The lesson was, that when you strip to a clean board...really strip. Look for the most unlikely...nuts and bolts between case and MB etc etc
Go down to CPU and memory only, then get rid of the memory next. One more step to a bare board.
Good luck LR