Hi @CJ Fig ,
Thank you for posting to Microsoft Community.
Based on your description, I understand there are some concerns about decommission Exchange Server.
Microsoft has an official document explaining How and when to decommission your on-premises Exchange servers in a hybrid deployment | Microsoft Learn which includes several common scenarios and indicates whether AD sync should be disabled.
You can follow the steps given in the document to decommission your Exchange Server.
If my answer is helpful to you, please mark it as the answer so that other users can refer to it. Thank you for your support and understanding.