Trouble booting up my Z370 Extreme4 Motherboard

Hi,
I configured and built my computer with the following configuration about an year and a half back. It has been running fine till recently.

Motherboard: Z370 Extrme4
Memory: CORSAIR Vengeance LPX 16GB (2 x 8GB) 288-Pin DDR4 SDRAM DDR4 2666 (PC4 21300) Desktop Memory Model CMK16GX4M2A2666C16
_Intel Core i5-8600K Coffee Lake 6-Core 3.6 GHz (4.3 GHz Turbo) LGA 1151 (300 Series) 95W BX80684I58600K
Graphics: EVGA GeForce GTX 1060 6GB SSC GAMING ACX 3.0, 6GB GDDR5, LED, DX12 OSD Support (PXOC), 06G-P4-6267-KR
Powersupply: Seasonic FOCUS PX-650, 650W 80+ Platinum Full-Modular
Storage: 2 M.2 1TB drives Corsair
Case: Fractal Design Define R6 Gunmetal Brushed Aluminum/Steel ATX Silent Modular Tempered Glass Window Mid Tower

ALl of a sudden, it would not boot up after shutting down. I would have to unplug the power cable and wait for a few minutes before I could plug in and everything worked fine. Within days even this stopped working. Everytime, I shut it down, it would not boot. The motherboard indicates it is getting power, but does not power up. Nothing happens when I press the start button on the case. I tried several things:

1. Sometimes, it would boot after disconnecting the power cable for a while and reconnecting it. This stopped working in the last two days.
2. I tried clearing CMOS after unplugging the cable. Worked a couple times, but if I shut it down, it would not work again.
3. I removed the video card and tried after keeping it unplugged for a while, no luck
4. When I got it up and running, I put the computer to sleep mode. When I woke up the computer, it showed it is powering up, but lost video on later and GPU. Had to shut it down but unplugging the power cable.
5. I unplugged the power cable, put the CMOS jumper on, removed the battery for an hour and put everything back, after removing the jumper. After a couple tries the machine booted up. I shut it down, and same issue again. It would not boot up.
6. I have tried the usual remedies - pull the memory cards out and re-plug them back in. Make sure that they are properly in place. I even tried booting up with a single memory card, but the damn thing wont even post.
7. When the machine was up and running, I updated the bios to the latest available 4.20. I have updated all the drivers and OS updates to the latest available. Of course they would matter only if the motherboard posts and gets to the OS. At least I thought flashing the BIOS to fix the issue, but it didnt.

I am at my road's end of everything I can try. I have built all my computers from scratch over the last 25 years, but never seen anything like this. I am suspecting I might have to RMA the motherboard.

I would appreciate if anybody can give me suggestions to try that I haven't done so far. I have also sent a message to the technical support, hoping they can offer some help.