Some more ideas:
- On the motherboard, are any of the capacitors bulging? They can fail without showing exterior damage though.
- Have you tried a different power supply?
- Have you tried reseating the RAM after putting things back together?
Check the Q-LEDs: https://www.asus.com/support/FAQ/1042678
Though per your last reply it sounds like they're all normal.
1) I don't see any.
2) I don't have another one unfortunately...
3) This seemed to have done the trick, I exchanged two RAM sticks from the same channel group and the PC started. I'm not really sure what happened or if it fixed everything but we'll see in time