Goal: We want to move to Exchange 2019, use a free Exchange license, and use Exchange 2019 to manage Exchange Online mailboxes.
You can simply install the latest Exchange 2019 management tools on any domain-joined computer (client or server) for management purpose, rather than installing a new Exchange 2019 server.
For more details please refer to these two documentation:
Manage recipients in Exchange Hybrid environments using Management tools
Install the Exchange management tools
One thing to note is that you can shutdown the Exchange 2016 server, but do not uninstall it.
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