How do you store webform submits to a different database?
General the most secure way is to push a webform submission into a different 'external' database is to create a webservice to populate the external database. Then you can use the 'Remote post' handler to push the submission data via the webservice into the external database.

@see https://www.drupal.org/docs/8/modules/webform/webform-videos#handlers

To disable the saving of submissions…


Goto /admin/structure/webform/manage/WEBFORM_ID/settings 
Check 'Disable saving of submissions'