Question PC died, EZ Debug VGA light on, tried a bunch of things, no clue what's wrong

[thenevernow]

Hi everyone, here's my sad story. There was a storm the other day and it did some damage to my PC, but 4 days and a lot of frustration later, I still can't figure out what's wrong.

Setup:
Motherboard: MSI MPG B550 Gaming Plus
CPU: AMD Ryzen 5 5600x
GPU: Sapphire Radeon 7900 GRE
RAM: 32GB (4x8GB) KINGSTON FURY Renegade DDR4-3200
PSU: be quiet! Pure Power 12 M 750 Watt (replaced with a Corsair Rm850X, see below)
Storage: WD Black SSD SN770 M.2 2280
Case (just in... case): Fractal Design Pop Silent Black Solid

The storm hit, the computer failed and turned off. I understand the damage could be limited to a cable or the whole thing could be fried, I accept that, stay with me.
The storm is over, the computer won't turn back on, I open it up and the EZ Debug light is on (solid white). Oh no, my GPU is dead! But I have an older GPU around, so before succumbing to despair I swap them and nothing changes. Ok, so maybe it's not the CPU. Other parts from my other PC are not swappable (too old and Intel-based), so I need fresh components.

First, I got a new MB, same as the old one. Exact same behaviour. Then the CPU (same one), still no luck. I thought "Ok, it's the PSU" and got a new one (different from the old one, see Setup above). Nothing.

Things I HAVEN'T tried yet:
- replace the RAM. I did however swap around the 4 modules I have. I can't be sure but they don't seem to be the problem. The EZ Debug RAM light never turns on when at least one module is installed.
- remove the MB from the case
- sacrifices to the Great Old Ones

Things I HAVE tried:
- verifying that the monitor the PC is connected to works fine
- replacing the HDMI cable
- using a DVI cable (with my older GPU) instead of HDMI
- replacing the power cable
- using a different wall power outlet
- booting without a monitor attached and with one
- removing the M.2 SSD or leaving it in place
- removing all MB-case connectors (fans, USB, audio). I even tested with no CPU fan connected but briefly, for obvious reasons
- checked that my older GPU works fine - it does. I know my CPU doesn't do video so I need an actual GPU in place. Put it back in the old system, all good
- tried the other PCI-E slot for the GPU
- cleared the CMOS (but probably irrelevant as I am now using a brand new MB)
- read all relevant conversation about EZ debug I could find here and elsewhere (nothing actually helpful, what I gathered is that the VGA light is supposed to also cover PCI-E and M.2, but is seems that people had it on and the issue was with the CPU or the MB itself or something else)

I hope I didn't forget anything. I know I still have a couple of things to try (replacing the RAM, mainly), but if anyone has any clue what's going on and what I should do, I'd be infinitely thankful.

Also, a couple of specific questions:

- is my PC (my MB, really) boot (to a BIOS error screen, at least) without a storage device attached? I'm assuming yes, but I could always be wrong
- same question, no monitor connected. Same assumption as above

Thanks in advance! 🙏