It appears you are trying to connect an Apple Pro Display XDR. Can you post the entire AGDCDiagnose output? It will show if Display Stream Compression (DSC) is supported and being used.
When DSC is used, the Apple Pro Display XDR only needs one DisplayPort 1.4 connection at HBR2 speed to support 6K 60Hz 12 bpc RGB. DSC has a 3:1 compression ratio.
DSC is supported by AMD 5300M, 5500M (16" MacBook Pro), W5700X (Mac Pro MPX module). It might be supported by your 5700XT and the new Radeon Pro W5700 also. In Windows, RTX cards also support DSC. RTX cards have a USB-C port so the GC-TITAN RIDGE isn't required. The W5700 also has a USB-C port but I don't know if it works in macOS. With the USB-C port, the USB functions of the display should work (brightness control, etc.)
For the USB functions to work with the GC-TITAN RIDGE connection, you need Thunderbolt PCI tunnelling to work.
Bandwidth comparison:
Code:
17.28Gbps 100% 4K 10bpc HBR2
25.92Gbps 150% 5K 8bpc HBR3
34.56Gbps 200% 5K 10bpc HBR2x2 Alpine Ridge
40.00Gbps 231% 6K 10bpc HBR3x2 Titan Ridge
51.84Gbps 300% 8K 8bpc HBR3x2
51.84Gbps 300% 6K 12bpc HBR2 DSC
Does SwitchResX show the 6K timing with 1290 MHz pixel clock?