AWS Pinpoint SMS - Setting to Avoid Error 429 "Too many requests"

I am using a GUI interface to send SMS via AWS Pinpoint. When I moved from 100 endpoint testing to 70% (approximately 7,000 endpoints), I received a 429 error, "Too mant requests." I am using a US TFN

About 1,200 messages were successfully sent, 5,800 generated the error.

Here are the setting I used:

Other than requesting an mps quota change, any suggestion on how can I adjust the settings available above to stay within the quota and not take 3-4 hours to send a single-part message?

Thank you in advance.