I know it would be a pain to do but you should pull the parts out of the case and install the GPU directly to the motherboard, see if that fixes the problem. Save you the trouble of replacing a good riser cable.
When I first got the M.2 I was still using my Ncase and bought it because I was seeing idle temps of 70C and load temps of 80C. At first I wasn't running a bottom fan and temps barely changed at all. I installed bottom intake fans for GPU cooling and later noticed my M.2 temps had improved a...
I have the same motherboard and the I am using a MyDigitalSSD which uses the Phison controller which run very hot. I installed a heatsink and had the bottom fan as intake, lowered temps 10 - 15C. Make sure you have a fan at the bottom, the heatsinks don't help if there is no air movement.
Not sure if any of the x570 will support Win 7, I have a MSI B450i and it supports Win 7.
Edit: Appears only works with * To support Windows® 7 64-bit, you must install an AMD Pinnacle Ridge & Summit Ridge CPU.
Nice build, I had the GTX 1070 Aero itx in my first MI-6 build, it's a good little GPU. If you want a custom 24pin cable you might have a talk with @pslate from pslatecustoms, he does a lot of custom cables for SFF cases and might be willing to do a custom fit one for you.
Maybe it is just me but I can't see the point to this case unless you are doing a full water cooled build and then it is brilliant. So I can't really see the need for 3 slot GPU support.
Also wanted to say that everything looks incredible so far and great job bringing it about so quickly.