Converted to a discussion. Simply changing the jdbc driver is supported see https://www.keycloak.org/server/db#_overriding_the_default_jdbc_driver

If the database is not SQL compatible (including DDL) with one of the built-in supported db types, you will have a problem.

If the database is not compatible DML-wise at the very minimum there would need to be a supported Hibernate dialect - but there currently isn't a supported first class, or direct quarkus, configuration mechanism to override the dialect.

I'm also not sure what would be entailed for customized liquibase support. An initial workaround would likely be to run all migrations manually - then you could modify them as needed to support your database.