Re: Do I Have to Delete last batch of data before POST next batch to an import?

No, you do not need to DELETE the export or sync resource related to the first batch before starting the second. 

The flow would be:

1. Create import #1
2. Create sync #1 referencing import #1
3. Start executing sync #1
4. When finished, check status on sync #1
5. Create import #2
6. Create sync #2 referencing import #2
7. Start executing sync #2
8. When finished, check status on sync #2

And so on.