--backup-dir is the correct command, and this is what I meant when I said folder.
If you want to view your google-trash then use:
--drive-trashed-only
This shows only trashed files. They still appear to be in the same place they were deleted from, so don't be confused by that. All files there should be normally accessible I'm pretty sure, regardless of being inside folders or not. If you want to restore something you can just move the files from the trash-remote to the Gdrive remote like you would move any other file.
You should set up a separate remote/mount for this trash-view and try your best to not confuse it, so choose a good name ect.