OK, the issue appears to have been the REDIS cache. After disabling it, and disabling all plugins and applications, switching to PHP 8.0 yielded a functional site. Finally.
Gonna leave it running (after I enabled the minimal plugins and apps) to make sure it holds up, and then will finally tackle the 4.7.6 update, followed by a switch to PHP 8.1 or PHP 8.2.
Hopefully this will be useful to others.
