URL "civicrm/a/#" breaks the UI
I have repeated the new install, this time including this:
composer config extra.enable-patching true
The site now completely renders UI contents, such as the new mailing page or the status page.
So then, going to the troubled upgrade site, having first configured the install with these composer configurations:
composer config extra.enable-patching true
composer config minimum-stability dev
composer config prefer-stable true
composer require civicrm/civicrm-asset-plugin
composer require civicrm/civicrm-{core,packages,drupal-8}
and then again:
composer update
composer civicrm:publish
The upgrade site is now working.