You cannot set the property applyImmediately, as it does not exist in CloudFormation.

Regarding to the CloudFormation documentation, changes to the DatabaseInstance are applied immediately: https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-rds-dbinstance.html

PreferredMaintenanceWindow.
This property applies when AWS CloudFormation initially creates the DB instance. If you use AWS CloudFormation to update the DB instance, those updates are applied immediately.

See also this quote from here: https://github.com/aws-cloudformation/cloudformation-coverage-roadmap/issues/597#issuecomment-1438357012

As of now, CFN applies all changes immediately: including AWS::RDS::DBInstance and AWS::RDS::DBCluster. Currently, there are no plans to expose this attribute as it would immediately conflict with the drift detector.