There appears to be a discrepancy in that you have setup the phones to sync with Onedrive, what you're looking for is a backup option. Judging by this article:
https://support.microsoft.com/en-ie/...rs=en-ie&ad=ie it should be possible to delete the photos on your phone after setting up the Onedrive upload. Other, smarter inputs required, as I don't have the answer unfortunately.