Maybe the RGB values you found for Thunderclouds are using a different scale but compared to the above chart it's definitely not 137,144,138, as Thunderclouds is quite a bit darker than Urban Obsession which is showing as 128,128,128 on the above chart. I think Thunderclouds must be around 110-115 average RGB values.
I just checked using MS Paint, pulling the colour cards from the Dulux site and the RGB for Thunderclouds is showing as 102, 106, 107. It's green heavy which ideally, you want to avoid (green is most visible to the eye in dark, hence night vision goggles/ red fades quickest and is hardest to see). I noticed that even +1/ -1 can have a slight impact on the shade of grey. But I guess it would be hard to know the difference to a near equivalent balanced RGB scale, for example 106, 106, 106, unless you compared them side by side at the same time.
I may have wanted something around 70, 70, 70 as per my original preference. However, after further searching the closest I could find was RAL 000 30 00 - Medium black from the RAL Design colour range, which appears to have an RGB of 68, 68 ,67, 68, 68, 68 or 71, 71, 71, depending on the source. However, my local Dulux Decorator Centre was not able to mix from such a code last time.
So, at the moment, I have settled on Forged Metal 26BB 10/008. On both wood and paper, it looks slightly more opaque and deeper than the S 7500-N. I also found the RGB using the colour card from the Dulux website to be 79, 80, 80 for Forged Metal. Where as for S 7500-N, I could not find an official website for the card, and got varying values across difference sites. That coupled with the fact that it looks better and preferable to my eyes in person make it my current choice.
In day light. Forged Metal 26BB 10/008 looks dark grey and in dim light with curtains drawn, it looks very dark grey. which I like. Whereas Fantasy Space 60RR 07/002 looks very dark grey in day light and then black in dim light, curtains drawn. So, contrary to wanting to try something close to 70, 70, 70, I feel like if I start going any darker, I may as well just go as black as I can all over my room. And the two main reasons I am not is purely because of the extreme ease of scratches, as I have read about, and secondly, not wanting to use the room as a living space still to relax, say with the lamp light on. All of that said, I am still wondering if there is a way that I can add something to the Flints Theatre Black to make it more durable. I remember reading someone provide a solution for the Rosco Velour Black on AVS, so I may check back on that.
As for vinyl matt being flatter than durable matt, since that was 2 years ago when you had that issue, and I can't notice any major sheen on my test patches, I can only assume they fixed the issue. But I will certainly have a word with staff in store to make sure before I might purchase the Forged Metal in Durable Flat Matt. And if I am still unsure, I guess I will just have to purchase a pot of vinyl mat and durable flat matt to compare.