@Electro Fan
Thank you for your response - I was unsure where to post this topic.
It might be too much of a niche's niche in the general computing sub-forum, the detailled signal integrity issues might also qualify it for the RF section?
Since I was unable to even find a single model of such a cable I wondered if it would be possible to solder one yourself...
...but is there a way to get a PCIe Gen4-quality M.2 M-key PCB where you can solder the ends of an SFF-8673-SFF-8673 cable to where one connector has been removed?