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Looks like a license cause, we can add a Windows 2019 Standard server to act as another DC.
But we need to buy CALs for our users. Essentials itself doesn't require CALs, however, when we introduce a full server, we will need to get them for all users since services, including DNS and authentication can be handled by either server.
For Windows Server Essentials, because of license limitation, it can only be deployed as Primary DC and holds all FSMO roles. If it do not met, licensing compliance component will force system shutdown.