Apex batch processing is not a good solution for a one-time operation inserting 6.5 million records from an external source.
You want the Bulk API, which is specifically designed for manipulating data via ETL in very large volumes. There are many Bulk API clients available, including the Salesforce Data Loader, Workbench, Amaxa (which I wrote), and various middleware platforms.
Most of the available tools expose very similar capabilities because they are all built on the same API mechanics. Note that if you choose to use the Salesforce Data Loader you do need to configure it to use the Bulk API rather than the SOAP API in its preferences.
If you have ongoing bulk data import needs, I would strongly recommend you evaluate middleware ETL platforms that can connect to the Bulk API.