For massive Gdrive->Dropbox transfers I would not over engineer it. Do basics and defaults. Use few flags to control speed not to hit Dropbox limits.
And maybe
--order-by size,mixed,75 --max-backlog 10000
to make all transfer more balanced.
max-backlog: controls how deep into filesystem rclone will scan to gather up a list of files to transfer. There is no need to do more.
order-by: those up to 10,000 items in the queue will be sorted by size and the threads will be filled with 75% small files and 25% big files. This helps the queue get saturated with a bunch of tiny files but also working on some larger files at the same time.