I'll try post a build log eventually, but thought I'd drop a note in that the Linkup PCIe 4 riser 25cm (Part #PCIE4EXT11SV-025) works to build the case in 3-slot but with the GPU in **1 slot** mode -- yes, weird, I know...
I've got a AMD WX7100 which is a single-slot graphics card, so set the case up with the mobo in "3 slot mode", by adding two 20mm standoffs to both sides of the GPU riser strut which I attach at the 2-slot position, which aligns the motherboard with the main spine strut in 3-slot position whilst pushing the riser mount point and therefore the GPU over another slot, so I've got a big space for activities behind it, to stick drives and other stuff in.
I've not seen anyone else run a 1-slot GPU, but doing it this way gives the GPU access to fresh air through the panel and hides the uglier drives and cables behind it. But yeah, for anyone unsure which of the many Linkup cables is the right length for this, it's the 25cm stright-out reversed-GPU-end one.