I resolved this by noting dbport no longer appears in config.sample.php, which says instead the port must be included in the dbhost config:
/**
* Your host server name, for example ``localhost``, ``hostname``,
* ``hostname.example.com``, or the IP address. To specify a port use
* ``hostname:####``; to specify a Unix socket use
* ``localhost:/path/to/socket``.
*/
'dbhost' => '',
Perhaps dbport was in my config.php because it was used in an earlier version of Nextcloud?
In any case, when I made that change to my config, it connected to the database correctly.