The Firefox console showed nothing, but the Chrome console showed the css was caught in a redirect loop. Turning off css consolidation brought the site back.
It's a hacky solution, but I added (temporarily)
if($name == 'field_yoast_seo_title' or $name == 'field_yoast_seo_description') {
continue;
}
to /web/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorageSchema.php:hasColumnChanges and everything ran as it should.
I had inherited the Varbase installation from someone else. When it was handed over to me, it was impossible to do anything with composer (including updates or new requires) because of a series of circular dependencies.
I brute forced by deleting the existing elements in the composer.json require and reinstalling current versions of everything, then running updb a few times. The site is now running nicely, but stalls on update 90006.