I have discovered that there is a second version of Magento 2 installed on my instance. This instance was moved from another host intact. Details are below.
I have Magento installed in the directory public_html\v23 (version 2.3.5-p1) and am upgrading to 2.4.3. All updates succeed, then when trying to access the UI or the admin panel, I get the following errors:
However, composer show amzn/* shows the later version. In EVERY case, the "database version" corresponds to the version displayed by composer show.
After some investigation, I discovered that there is an installation of Magento in the public_html directory. Composer show in THAT directory shows versions equal to the "code version" shown above.
It would be simple enough to upgrade the public_html version to 2.4.3, but I could also remove it.