Hi @Bruce Richards,
Thank you for posting to Microsoft Community.
Based on your description, I understand that you got concern about how to remove hybrid configuration when all your mailboxes in Exchange Online and you don't need to manage my users from on-premises and no longer have a need for AD.
You could refer to the Scenario one of How and when to decommission your on-premises Exchange servers in a hybrid deployment | Microsoft Learn and follow the steps given to decommission remove the hybrid configuration.
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.