Hi Partner,
As per my understanding, it seems that you are worried about some of clients among 20000(or massive) clients may have same ISP?
so you want all mails could be sent gradually during a period?
As your example, if we set 1 hour as interval, then 2500 mails will be sent per hour in whole 8 hour process?
If so, unfortunately we couldn't control marketing email sending process.
(Emails will still be sent from MS server even we have set DKIM, our domain is just linked to CRM from this option. However, by enabling this setting will improve our deliverability from official doc, you could contact MS support for how do they protect their(or your) IP reputation and manage balance).
In a standard setup, Microsoft implements SPF(Sender Policy Framework) for you and does what it can to maintain a good sender reputation for all Microsoft sender IPs.
In addition, you could take those integration of Dynamics and Mailchimp as reference:
https://zapier.com/apps/mailchimp/integrations/microsoft-dynamics-crm
https://www.powerobjects.com/powerpacks/powermailchimp/
If not, please let me know your real requirements. :)
Regards,
Clofly