The 429 errors are amazon drive’s way of telling you to slow down.
To get the best transfer you’d run as fast as you can without getting too many 429 errors as rclone has to wait before retrying a 429 error - there is an official spec for this as part of the amazon drive API docs which rclone has to adhere to (part of getting rclone accredited for Amazon Drive was getting that tested).
I know that Amazon Drive has various limits, per user, per app etc, but none of those are public information, so you’ll have to experiment and see what gives you the maximum throughput.
It will also depend on whether you are sending lots of small files or a few big ones.