Submit documents to approval workflow then to a library

is there an easy way to create a workflow or form that lets users fill out the required metadata, attach the document and after it is approved, get it submitted to a library?

Yes, and this is the more common approach to create a 'drop off' library or list when a permission barrier exists for users. If you have detailed data required that would necessitate a Form, then you will have to copy the attachment to the item. This method is slightly complex and would require a web service or REST call. Or, you could simply copy the item into another permissioned list that includes the form with its attachments.Turning attachments into working, controlled documents is a good walk through of setting up the web service call. One of the beneficial side effects of using this method is that the web request action requires a username password. You can use this to your advantage and copy the item when the current user doesn't have permission to do so on their own.

If you use a Library with no start form(like using lists with attachments), then you can more simply use Copy or Copy to SharePoint action.