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Maximus 8 Ranger - Q-Code 18 with BIOS update

Level 7
Short version;
Updating BIOS results in Q-Code 18 ("Pre-memory System Agent Initialization is started") and no boot.

Long version back story;
Some years back (estimated 2018) this board ran just great with a relatively recent BIOS (with the same other hardware).
I've had the system set up with a win7/win10 dual boot and all was just dandy.
At some random day, win10 decided to BSOD while I was playing a game and afterwards the OS was corrupt.
Shrugged it off and kept running win7 without issues.
Some months later, after turning off the machine in an evening, the next morning the machine no longer booted up and showed Q-Code 18.
Tried the usual troubleshooting to no avail, gave up and ordered another Maximus 8 Ranger. Swapped everything around, it booted.. yay!
As the BIOS was from 2016, figured I would update it. Doing so again gave me Q-Code 18 fearing I broke the new board I just bought..
Downloading an older BIOS from 2016 and using the Mobo's BIOS Flashback function (amazing feature!), got it back to working normally.
With more and more new games requiring win10, I attempted to re-install it, but now every time I try this, the pc will reset in the setup, at random points (it's like you push the reset button).
I fear the 2 things may be related.

I've tried the usual trouble shooting for the Q-Code 18 issue (single RAM module, other RAM from a different machine, unplug all unnecessary things etc etc) and it still occurs even as recently as a few weeks back. And why wouldn't it.. nothing has changed haha.
As for the win10 setup, I've also tried a completely seperate SSD with nothing on it without any other drives connected, no change.
Tried installing it from DVD and USB, no change.
Win7 currently still runs just fine.

The system is made up of the following parts;
Asus Maximus VIII Ranger (BIOS ver 1402) (tried many different versions, anything newer results in Q-Code 18)
Intel I7 6700K (supported since BIOS 0401 according to site)
2x 8GB Kingston HyperX 2400Mhz
Samsung 960 Evo 500GB M.2 NVMe SSD
Gigabyte G1 GTX1080
WD Black 3TB HDD
Seagate 1TB HDD

See this pic for further BIOS details.

I really hope someone has more info on this (for me) extremely strange problem, it's driving me absolutely mad.