I think we are up and running again! I got the forum software running, then updated to the latest version, then re-enabled PHP 7.3, cleared some cookies and fixed a failed MySQL import. Things are finally where they should be, possibly excepting some forum customizations that I have to re-install.
(BTW, there were several posts that helped track down the PHP error log issue. Between my misunderstanding of some of them early on and the fact that more than one approach was viable, I am not sure what to mark as a solution.)