So for those searching, I've been trying a multitude of things; again thanks to all that contributed on this thread.
Ultimately, I contacted Intel and Asus. The latter haven't gotten back to me yet (as it's gone to HQ), but Intel took some time with me to explain the OC.
They told me to keep things simple and use offset voltage rather than manual Vcore (as per the suggestion of 95% of videos/guides), the result has been great and actually lead me to a stable 4.7Ghz overclock at a lower voltage than what I had when I was on auto.
The steps are extremely simple - for an Asus board, here are my settings:
-AI overclock tuner: XMP 1
-Asus multicore enhancement: Enabled (for some reason, disabled which is normally suggested dropped my CPU frequency in non-AVX loads down to 4.3GHz)
-AVX instruction: set to 3
-CPU core ratio: Sync all cores to 47
-CPU core voltage: Offset, negative sign, -0.020
And that's it. No fiddling with LLC, SVID, manual Vcore or any other setting. Of course, if you want to set your BIOS preferences (ie fan speeds, LEDs, C-states etc, go ahead)
I then (as per Intel's suggestion), ran RealBench for 8hrs. Avg temp sat at around 78.8c with a Vcore of 1.128v and VID of 1.2390v.
So what's the difference between this and my Auto setting, I achieved before?
The Auto setting whereby I had everything in the BIOS set to auto, apart from XMP1 and/or manual RAM timings, yielded the same results (4.7Ghz), but at 1.243V and a VID of 1.3367v; this gave off an avg temp of 83c.
Ultimately, I shaved off a few centigrade and managed to reduce my Vcore by 0.115v. Decent.
Now, why didn't I push further?
Well, I actually did - I went up to 4.8GHz, whereby I reached almost a +/-0 offset, and went up to 5Ghz where I was hitting more of a + (not minus), 0.050 offset. The result of these two yielded really high temps - whereby I wouldn't feel comfortable running that 24/7 - I'm speaking high 90s, close to 100c at the 5GHz OC, and that was after only 10mins of RealBench, let alone 8hrs, which I didn't/want to test.
So what made me decide on 4.7Ghz, well a few things:
-The actual benefit of having 4.7Ghz vs 5Ghz in synthetic benchmarks is absolutely minimal; to give you an idea, I've 20 separate benchmarks from various benchmarking software
-The silicone lottery, had I had a better chip, I'd have been able to achieve a higher OC
-The temps, with my Noctua D15 air cooler, there's only so much I can achieve with the fans running at 1,500 RPM for 8hrs; the i9 9900K is a hot chip and is power-hungry if you push it - temps can easily get into the mid 90s on a 'low' OC - dependent on silicone lottery, of course.
Anyway, tl'dr, I got unlucky with silicone lottery, and must have one of the worst i9 chips out there, but I don't really care - as I'm still hitting fantastic results at a lower-mid-tier OC
I thought to share this, in case anyway was wondering where I got to.
I'll be sure to share more thoughts if and when I have them.
EDIT: Added bull BIOS screenshots.