Simplify your life so that the next time you do this it's not manual. Create a file that contains something like:

embed-server --server-config=standalone.xml --std-out=echo
    
batch

#
# remove the default provided datasource
#
/subsystem=datasources/data-source=KeycloakDS/:remove

#
# add the module
#
module add --name=com.mysql --resources=${user.home}/Downloads/mysql-connector-java-8.0.20/mysql-connector-java-8.0.20.jar --dependencies=javax.api,javax.transaction.api

#
# create the driver
#
/subsystem=datasources/jdbc-driver=mysql:add(driver-name="mysql",driver-module-name="com.mysql",driver-class-name=com.mysql.cj.jdbc.MysqlXADataSource)

#
# create the datasource
#
/subsystem=datasources/data-source=KeycloakDS/:add(connection-url=jdbc:mysql://localhost:3306/keycloak,driver-name=mysql,jndi-name=java:jboss/datasources/KeycloakDS,initial-pool-size=4,max-pool-size=64,min-pool-size=4,password=keycloak,user-name=keycloak)

run-batch

Run this with $KEYCLOAK_HOME/bin/jboss-cli.sh --file=<the_file_name>. Keyclock cannot be running when you run this script. And the user and database must already exist before you restart Keycloak.

What this does is

1. Remove the "old" Keycloak Datasource
2. Install the module. In the code above you will need to update the location of the MySQL driver.
3. Re-install the Keycloak datasource with the correct parameters.

The advantage is that this is repeatable if you need to install again or into a different server.

And do you need the XA driver? If I was developing in Wildfly for myself I might need XA but I don't think Keycloak is going to take advantage of it.

Try this:

Put the connector in /standalone/deployments then start the server again. It will deploy and register the connector in the system.

Next add this to your standalone.xml datasources.

<datasource jndi-name="java:/jboss/datasources/KeycloakDS" pool-name="KeycloakDS" enabled="true">
                  <connection-url>jdbc:mysql://localhost:3306/keycloak?useSSL=false&amp;characterEncoding=UTF-8&amp;useUnicode=true&amp;useJDBCCompliantTimezoneShift=true&amp;useLegacyDatetimeCode=false&amp;serverTimezone=Africa/Luanda</connection-url>
                  <driver>mysql</driver>
                  <pool>
                      <min-pool-size>5</min-pool-size>
                      <max-pool-size>15</max-pool-size>
                  </pool>
                  <security>
                      <user-name>usernameforthedb</user-name>
                      <password>password</password>
                  </security>
                  <validation>
                      <valid-connection-checker class-name="org.jboss.jca.adapters.jdbc.extensions.mysql.MySQLValidConnectionChecker"/>
                      <validate-on-match>true</validate-on-match>
                      <exception-sorter class-name="org.jboss.jca.adapters.jdbc.extensions.mysql.MySQLExceptionSorter"/>
                  </validation>
              </datasource> 

Change your serverTimezone accordingly. And, for safety, add the database in mysql before starting the server.

Restart again.

P.S. keep this whre it is of course:

<driver name="mysql" module="com.mysql">
     <xa-datasource-class>com.mysql.cj.jdbc.MysqlXADataSource</xa-datasource-class>
</driver>