Thanks for your feedback and for offering your support!
Unfortunately I was not able to find a GTX 970 that I had reliable information would work with my Dell UP2715K@5K@60Hz.
So I needed to go GTX 980. I found a great deal for an open box EVGA GTX 980 on Black Friday, so I ordered that. (for reference: https://www.amazon.de/gp/product/B00O3JZAYA/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1)
Thanks for the Info. I will not overclock the graphics card. It is only being used for video production, memory bandwidth is important in that case and I guess the GTX 980 220GB/s should be sufficient for quite some time.
In the meanwhile I got my custom cable (shipping from North Carolina, USA) and the Dell DA-2 power supply.
Unfortunately the colours of the wiring (black/yellow) / the placement of the paperclip seems to differ from the diagrams that can be found in the beginning of this thread.
There is few different ways on how I could connect the adapter cable to the PSU. None of both variants is identical to the image in post #1
Would this still work? Am I missing something? Or is the adapter soldered incorrectly?
UPDATE: I contacted the eBay seller, he told me I got a wrong cable indeed.
Variant 1:
Variant 2: