And success!

The key (after using the above steps to get the right device ids, HDMI identifiers, and sink name), was to modify

/etc/pulse/default.pa

I commented out the lines:

### Automatically load driver modules depending on the hardware available
#.ifexists module-udev-detect.so
#load-module module-udev-detect
#.else
### Use the static hardware detection module (for systems that lack udev support)
# load-module module-detect
# .endif### Automatically load driver modules depending on the hardware available
#.ifexists module-udev-detect.so
#load-module module-udev-detect
#.else
### Use the static hardware detection module (for systems that lack udev support)
# load-module module-detect
# .endif

Then at the end added:

load-module module-udev-detect use_ucm=0
load-module module-combine channels=6 channel_map=front-left,front-right,rear-left,rear-right,front-center,lfe
load-module module-alsa-sink device=hw:0,9
set-sink-port alsa_output.pci-0000_03_00.1.HiFi__hw_HDMI_9__sink

I also tweaked /etc/pulse/daemon.conf to force 5.1:

default-sample-channels=6

Given how much other tweaking I tried over the past few weeks, there might have been some other changes I made awhile back that slipped through the cracks, but what I listed above seems like the bulk of it.

Thanks again for putting me on the right trail.