Stuck in Preparing-data-migration status while restoring rds db snapshot from s3

You can't restore exported snapshot data from S3 to a new DB instance. Please see the limitations of DB snapshot exports.

DB snapshot and Snapshot export are not the same. You can't use the snapshot exports for restoring a DB instance. When you export a DB snapshot, Amazon RDS extracts data from the snapshot and stores it in an Amazon S3 bucket. The data is stored in an Apache Parquet format. The exported data can be accessed by analytical tools like Amazon Athena or Amazon Redshift Spectrum.