Hi there! Welcome to our community! My name is Alexi and I am the PM for Atlas SQL and the BI Connector. If you have your data within Atlas, our Atlas SQL Power BI connector is a great way to connect. As you mentioned, you could replicate your data from EA on prem to Atlas for the purposes of performing various tasks, including analytics. I have a few customers that are doing this today. They use mongo mirror to accomplish the replication.
Alternatively, you could connect from your on-premise EA MongoDB cluster to Power BI using our self hosted on prem BI Connector and the option for ODBC connection. This method of connection (via MongoDB BI Connector) will be supported, but will eventually be replaced within the next few years.
Hope this helps on your analytic journey!