THen you should search for something outside of mailcow, maybe an additional MTA.
Or you could do some scripting:
http://www.simonecaruso.com/limit-sender-rate-in-postfix/
Postfix ratelimits I think do not survive a reboot. When Postfix reboots, counts start at zero again. Correct me if I am wrong.
So setting up something per month would not be a good idea.
I guess you have a 20K limit because you are using a relay service?
Then calculate how many users you have and what each of the users can send per day to not reach the total of 19K. So you would have a buffer if there is a reboot in one month, resetting the counters for worst case one whole day.