Currently there is smallrye-config 1.6.2 which does not contain the highly appreciated feature of variable expansion. Please bump to the latest (1.9.0) smallrye-config implementation.

See: https://github.com/smallrye/smallrye-config/releases

Adapt https://github.com/wildfly/wildfly/blob/20.0.1.Final/microprofile/config-smallrye/src/main/java/org/wildfly/extension/microprofile/config/smallrye/deployment/SubsystemDeploymentProcessor.java#L57 accordingly to comply with https://github.com/smallrye/smallrye-config/blob/1.9.0/implementation/src/main/java/io/smallrye/config/SmallRyeConfigFactory.java#L52.