https://forum.xda-developers.com/showpost.php?p=83831075&postcount=13 is a thread which purports to make available 1.5GB .IMG file flashable with AMLOGIC burner utility. Quite interesting as Xiaomi has NEVER made publicly available their rom image files.
The zip file was obviously sniffed from a recent OTA update for PIE on MDZ-16-AB
I have absolutely no involvement in making these files public and make no claims as to their viability or functionality.
Explore entirely at your own risk.
Sorry, I don't have access to a working MiBox to experiment with these days. Perhaps one of you brave souls will play and report your efforts.
The referenced update zip file is rather mundane. It is simply AndroidTV Oreo v8, build 2562, which was released in April 2019.
I think the process is to burn the .img file with Amlogic Burning Tool, which will revert to an earlier version of Oreo.
Thence flash with update zip file to Oreo build 2562. Wait for a more stable version of Pie and go from there.
Waiting for you adventurous MDZ-16-AB owners to give it a go...