Alan Pitthan Thank you for reaching out.
My understanding is that you are trying to build a connection between Mongo and Cosmos then configure a Vnet gateway and build a Power BI to create a data flow from both ends and make a refresh live data on PBI reports. Please let me know if my understand is not correct.
"However, I don't want to risk unnecessary costs by doing the wrong setup and I'm not even 100% sure if this setup would actually work to achieve my goal".
It will work for your scenario.
You can use Azure cosmos DB as an intermediate between mongo DB and PBI. Once the data is in sync between Mongo DB and Azure cosmos DB then you can set up Synapse link on the Azure cosmos DB to see the real time data.
for synapse, we can use either Azure SQL or on-prem SQL to setup the synapse link. (Real time data)
Important Note!!!!!: The PBI portal experience isn't there for mongo yet. So, just follow the instructions not using the portal experience.
Enabling Azure Synapse Link has cost implications. See Azure Synapse Link pricing section for more details.
When to use Azure Synapse Link for Azure Cosmos DB?
There is a great demo on YouTube by Our Azure Cosmos DB product group on how to use the Azure Synapse link for Azure Cosmos DB to see real time data in Power BI. https://www.youtube.com/watch?v=U6K9P-1snd0
Currently you can't enable Synapse Link on your existing MongoDB API containers. Synapse Link can be enabled on newly created Mongo DB containers.
Please let me know if that answers your question.
If it does, please don't forget to mark as accept answer. This will be useful for others community members with similar ask.
Another example here but with python in case you want to explore https://github.com/Azure-Samples/Synapse/blob/main/Notebooks/PySpark/Synapse%20Link%20for%20Cosmos%20DB%20samples/E-Commerce/spark-notebooks/pyspark/01-CosmosDBSynapseMongoDB.ipynb