Posting a question & answer to share my hardships (and solution) with the internet. I hope others in this situation see this post and are able to save hours of their life on needless POST diagnostics.
ASRock currently documents on their A320M Pro4 web page that the MOBO is compatible with Ryzen 5 3600 CPU with P5.90 BIOS version. THIS IS INCORRECT (or, at least, is incorrect for the latest off-the-shelf Ryzen 5 3600 CPUs). You must use BIOS version P6.00.
That is all.