Simplest solution would be to have a copy of your Source DE and then use a SQL Query to do a full copy of that DE to your copy DE.
/* Target: copyDE */
/* Action: Update */
You then would run a second SQL query to overwrite your Source DE to 'clear' it of all the info:
WHERE 1 = 2
/* Target: SourceDE */
/* Action: Overwrite */
by setting the condition 1 = 2, you will guarantee a return of 0, which means that the overwrite will delete all records and add in the results (none) from your query - therefore a duct tape 'clear' of sorts.
This will allow you to keep a sort of archive of your Journey data in the copyDE without bogging down your SourceDE.
One thing of note though is that any emails sent may have issues with the VAWP if deleted from the Source DE. I am not sure, since they are going through JB, if this is applicable (I believe the VAWP is based off Journey Data, not Contact Data, so it is stored temporarily outside the DE itself.)