Temps look good, I'm assuming they're for under load?
What's the issue with the cable management? Is there not enough space for the cables as they are now, or do they not bend into the positions you imagined for routing?
For the GPU mounting, I think most of them have something for the riser cable to screw into, so the PCB can add support, plus the retention screw by the back plate. Is there a spot for the retention screw on the back plate?
As for SSD placement, behind the GPU might not be the best place given how hot that can get and the GPU issues you're having. Is there enough space on the front panel to mount them there?
The case itself looks really good, I especially like the way the RGB shines through.