I’m able to connect to the client’s MySQL DB as a data source, no problem. They’re running MySQL 5.5, so it looks like the issue’s been that CF11 needs an older JVM that can’t connect to the latest version of MySQL (8.0), and as Brad noted this isn’t a Commandbox issue. Rather than try to replace the MySQL driver in cfusion/lib with a more recent one, which has errored for different reasons, I’m going to see if MySQL 5.5 is available for DL anywhere. Thanks much to @jclausen and @bdw429s for your help on this.
Later - resolved the issue by downloading the same version of MySQL that the client’s running (5.5.50), and setting it to port 3307 so I can keep the more recent instance of MySQL on my machine (8.0.33) on port 3306. Older versions of MySQL are available for download going way back, and it’s much simpler than swapping out .jar files to try to make CF11 able to connect to MySQL 8.