The iMac Pro does have Alpine Ridge. Scroll to the first NHI in the parsed ioreg output and lookup 8086:15d2 in the pci ids database. Scroll up to the upstream bridge which is 8086:1578 and the downstream is 8086:15d2.
The EDID for the display says it support RGB and 4:2:2 but says nothing about 4:2:0. I suppose you could try a mode between the HBR2 RGB 8bpc max (720 MHz) and the HBR2 4:2:2 8bc max (1080 MHz) to see if macOS is smart enough to add a 4:2:2 mode.
It says it supports modes with refresh rates from 24 to 75Hz but then says it supports refresh rates from 56 to 76Hz which is wrong since you are using 6.1K30Hz.
The EDID has the resolution of the XDR (6016x3384) and the 5K but they are only at 60Hz. For 4K it has a 60Hz timing and a 30Hz HDMI timing.
I can't tell if the display supports HBR3 or not. 02201h says HBR which is wrong since you are using HBR2. I guess we need a DPCD from an Intel Mac that supports HBR3 to check this (maybe the Alpine Ridge is affecting 02201h). The display might be limited to HBR2 since that's all that's required to support 6K60. But the user manual says it supports HBR3. I suppose if you had HBR3 then you could get RGB 10bpc at 30Hz instead of the 8bpc that you are getting now or you could get 49Hz RGB 8bpc.
IOFBTimingRange of the GPU says it can support up to 1080MHz which is the limit of HBR3 8bpc but maybe the GPU doesn't know about Alpine Ridge. 1080MHz is also the limit for HBR2 4:2:2 8bpc but IOFBTimingRange says it only supports RGB, 4:4:4, and 4:2:0. IOFBTimingRange says the GPU doesn't support DSC as expected.
The DPCD shows the display supports DSC. EDID would mention DSC only for HDMI 2.1 inputs. An AllRez for that connection would be interesting. HDMI might include 4:2:0 modes.
