Fantastic job team, it worked!
I flashed the image as provided and imported a backup configuration to get some media ready, I found I still had to add the dtoverlay in the boot config to detect the soundcard but afterward I could then select "ALSA: snd_rpi,justboom_digi, S/PDIF" as an output which was a big clue to tell me things were working.
A few more settings later I could now select the passthrough device also as "ALSA: snd_rpi,justboom_digi, S/PDIF", I also found I had to enable AC3 transcoding in order for AC3 and other 5.1 media to properly go to each speaker.
Then the moment of truth...
Ta-da! We get signal!
Settings configured as follows;