onSuccess/onFailure are primarily for status reporting. DPL doesn't really support the idea of branching or error handling. You set up your graph, and it runs through step by step. If something fails, it either pauses or aborts.
I would create a new activity that would run after your copy, using the dependsOn property:
{
"id": "DeleteDataStep",
...
"dependsOn": { "ref": "CopyRdsToS3Step" },
...
}