I used the Google Drive app (or Backup & Sync, or whatever it's called this week).
After the Google Share was created I used robocopy to copy the files into the Google Share via the Windows Google Drive App. Just keep an eye on the system tray icon to see when the files are all done syncing. We have a dedicated user account that we use as the owner of all our shared drives, almost like a service account. This helps us keep things organized.
I migrated tens of thousands of files into Google Shared drives with this method when the world shut down for COVID.
I can share some more details if interested.