Update: Still not working, but getting closer.
Using 127.0.0.1 as my DB host (instead of localhost) in the NC installation wizard seems to get me closer to a solution. This was discovered on this StackOverflow answer.
Additionally, I discovered I needed to append the correct port to the host address, e.g. 127.0.0.1:3306.
I created a nextcloud database in MySQL.
CREATE DATABASE nextcloud;
I then created a new DB user and granted the user access to the DB.
CREATE USER '<user>'@'127.0.0.1' IDENTIFIED BY '<password>';
GRANT ALL ON nextcloud.* TO '<user>'@'127.0.0.1';
Even after running the query, though, I get this error in the NC initialization wizard:
MySQL username and/or password not valid You need to enter details of an existing account.