To migrate to Azure and decommission the on-premises data center, you would need to create the 5,000 user accounts in Azure Active Directory. The easy way to do this is to sync all the Active Directory user accounts to Azure Active Directory (Azure AD). You can even sync their passwords to further minimize the impact on users.
The tool you would use to sync the accounts is Azure AD Connect. The Azure Active Directory Connect synchronization services (Azure AD Connect sync) is a main component of Azure AD Connect. It takes care of all the operations that are related to synchronize identity data between your on-premises environment and Azure AD.
https://docs.microsoft.com/en-us/azure/active-directory/hybrid/how-to-connect-sync-whatis

Reference:
https://docs.microsoft.com/en-us/azure/azure-resource-manager/resource-group-overview
https://www.codeisahighway.com/effective-ways-to-delete-resources-in-a-resource-group-on-azure/

A resource group is a logical container for Azure resources. When you create a resource group, you specify which location to create the resource group in. However, when you create a virtual machine and place it in the resource group, the virtual machine can still be in a different location (different datacenter). Therefore, creating multiple resource groups, even if they are in separate datacenters does not ensure that the services running on the virtual machines are available if a single data center fails.
https://docs.microsoft.com/en-us/azure/azure-resource-manager/management/overview#resource-groups