I have only run this engine for a few minutes (several short runs) in the garage before the hex drive broke.
Since the oil was still cold, it made plenty of oil pressure. This pump and relief valve spring typically made 60-65 psi with warm oil and higher than idle type RPM.
I didn't notice any oil pressure issues until it went to zero.

I expect to start it again on Wednesday. The investigation is done, the new bronze gear oil pump drive shaft is installed, and I put a new oil pump in just in case.
I primed it with a drill late today (Tuesday) and it all seems normal.