Moving document library

Hi,
Your options here are the following:
1) Use a trial version of a migration tool that should help you here on the move (Sharegate, Quest, AvePoint)
2) Write a PS Script that makes the move
3) Syn. your document library content to your local PC, create the document library in the subsite and then sync back the content to this new document library or use the SharePoint Migration Tool as an alternative

If you want to keep metadata, versions, etc I would recommend option 1)