It's only a 5mm long standoff, I don't know if shorter M3 standoffs are common. However, you could probably 3D print a spacer between the riser and the back of the motherboard panel and use a slightly longer through-bolt.
FYI, got the custom cables in and finally got the case put back together.
CPU is a 4770K, stock, with the 645LT and a 25mm thick fan using the Losercase 3DP mods
GPU is a 3080 FE, running at 1875mhz@ 850mv.
PSU is in the normal orientation.
Running Heaven on a loop, GPU settles around 75C. I don't have an accurate way of measuring sound, but the case is clearly audible but not loud.
Running OCCT and P95 simultaneously, GPU settles around 85C, CPU bounces around in the high 80s for the most part. Case is definitely loud at this point, most of the fans are running at 75%+ speed.