Thanks for this workflow, @vitor!
I had been clinging on to my Drive Mirror folder since it was still indexed by Spotlight, but I can now finally consider return to the promised land of File Streaming.
For others in a similar situation, it seems the default (or default back then) path for the Google Drive Mirror folder was in /Users/[youruser]/Google Drive (with a space).
Although I don't know if changing from Mirror to Stream, which I suspect will change the path to /Volumes/GoogleDrive (since it's the workflow default), would also move or discard the Mirror directory completely.