I had good luck with using an older version. Worked right away as soon as I downgraded to deepquestai/deepstack:gpu-2021.09.1. I know ow it's not the same setup as yours but I was experiencing the same issue.
I am in the same boat. Considering a p400 for BI and keeping iGPU for Plex transcoding (Unraid, i3 UHD iGPU). Curious if you did use a P400 or similar for your BI setup and if it worked ok? Thanks!