hi,
@Animosity022 might have some suggestions about migrating from gdrive to dropbox.
much depends on your internet speeds when transferring large amount of data up and down simultaneously.
once you have maxed that hard limit, tweaking rclone will not help.
i had a recent need to move a few TB from wasabi to aws s3 deep glacier.
i rented a cheap cloud vm, ran rclone on that.
as for your command, might test by increasing --transfers and --checkers
and this topic might be a good read.
https://forum.rclone.org/t/sync-not-removing-deleted-files/30865