I would say cut out the middle man. Connect the legacy Oracle database directly to your SAP system and use EXEC SQL to read the data. With data volumes that large it's best to have a so direct path as possible. This will also make post load validation a breeze.Then after the conversion you can remove the connection.
A quick note on BODS, you can call BAPI's from it so you can also create your own RFC enable FM that does the table updates for you, no need for IDocs that way.