I think thats a very good summary, thanks for explaining all of that. The last point you make is key, stability has to be number 1, i have been having crashes for a long time and it's because my ram has been unstable ever since i installed windows about 6 months ago and swtiched on the XMP profile. Everything i have since installed has probably had some errors baked in due to the ram instability. As you say, thats really incidious to the system and litters everything with errors and these build up more and more over time.
Tomorrow i am taking off all ram overclocks and installing windows and all drivers again from scratch and then only using the 3600Mhz profile thats passed a full memtest, (or only use other profiles i have proved are 100% stable in memtest). I think i just took XMP as a gaurentee and when i saw 4000Mhz displayed in windows assumed that would be stable and never thought to test it.
My mother board is not anything super high end, because i am using mATX and there are only a small choice in this space. Definately very ambitious of me buying such high frequencey ram even though i saw the motherboard spec saying it could suppprt up to 4133, but when you look at the QVL list they only have 1 set of ram listed as tested at that speed! Probably just to be able to market the board as "speeds up to 4133Mhz". I will be happy if i can push the timings down very low on 3600Mhz, as you have said, real world performance is not massive when going up to the super high frequencies. I have seen lots of charts saying ram speed is more or less irrelevant when gaming at 4k as its all VRAM on the GPU at that res.