If you want to migrate your Windows 2019 domain controllers in the cloud, you could deploy domain controllers in Azure Virtual Machines and replicate the on-premises via VPN.
Hybrid setup lets you bring your existing on-premises Windows Server and SQL Server licenses with active Software Assurance or subscriptions to Azure
You also will have 180 days of concurrent use rights of SQL licenses, while migrating servers.
You may refer the following links for more details
faq
hybrid-use-benefit-licensing
You can purchase Office 365 licenses for the clients, if you don't have these available yet
For joining the workstations to Azure AD, you can go for either of the three methods below: