> doesn't honor dropbox's retry-after header

That hasn't been true for more than 8 years now.

Source: https://github.com/rclone/rclone/blob/9abf9d38c0b80094302281...

And the PR adding it: https://github.com/rclone/rclone/pull/2622

I honestly haven't used it with Dropbox before, have you tried adjusting --tpslimit 12 --tpslimit-burst 0 flags? Are you creating a dedicated api key for the transfer? Rate limits may vary between Plus/Advanced forum.rclone.org is quite active you may want to post more details there.