Exactly the same here.
I also opened a working device for NvME (WD) update with a Sintech adapter. I also strictly used to be electrostatic discharged. I did not do a CPU replacement. After connecting all components back the iMac did not give any signs of life. All diagnostic LEDs were off. Used a known working PSU did also not changed the behavior. I also replaced Hardware and build up many Computers in my life but Apple seems to be extremely fragile…