So the last few days have been a bit chaotic..
I know this will be of little comfort but...I'd like to add that you're not crazy

I too have been struggling with my board the last week or so on and off and I am in exactly the same position. Seems no matter what steps I follow, just can't get the system to POST/No Output on VGA or H5 Viewer etc.
Like you I've tried:
- Multiple combinations of firmware updates, BMC updates (including the 12.61.35) and irrespective of the success messages, I'm still not able to POST
- Multiple combinations of RAM in various configurations (I don't have ECC only 4x 8GB DDR4 Corsair)
- Switched out the PSU: 1st EVGA 450B3 /2nd Coolermaster 500W /current Coolermaster 650W
- Removed all additional components - no NVME, no SATA connected, even pulled the USB case header leads, leaving just the case power buttons etc.
- Removed and reseated the CPU cooler (Gelid inc. in the bundle) and checked the pins on the 4650G before refitting
- Removed the CMOS battery a couple of times, shorted the CLR_CMOS pins half a dozen times. Fitted a new CMOS battery yesterday.
I appreciate that perhaps I was a little naive in thinking I could get a server going with just the board and a collection of spares from years of building gaming machines for my sons, but this is infuriating.
I've got an order out for some Crucial RAM as obviously Corsair aren't on the QVL but aside from buying another CPU I'm quickly running out of ideas. Unfortunately I don't have another AM4 system to try the parts with so it is possible, that my 4650G is faulty and I may have to try and source something like a 3600 locally.
Keep yer' chin up and if I do manage to get it working I'll come back and share!
Richy.