I ran into the same problem a couple of months ago when I had to consolidate all my cloud storage accounts for a new workflow. While researching, I found this guide transfer google drive to onedrive and it explains how to transfer Google Drive files to OneDrive without manually downloading and re-uploading everything. What I liked is that CloudMounter mounts both services as local drives, so you can literally drag files from one cloud to the other as if they were regular folders on your computer. It saves a crazy amount of time.