What do you mean by "traditional video calling"? Are video calls not encrypted? Is traditional meaning out of the box video calling? What is the alternative?
Disclaimer: I am not a synapse dev, but I have run a non-federated server for a long time.
Assuming you have fully disabled federation and Matrix.org provided services, data should not be leaving your instance. I run mine with bridges for services like Signal, IRC, and Discord, so as expected data goes out through the bridge software (via VPN).
To be extra safe, you could run it in a Docker container who's network is limited to local, and an nginx proxy for Element.
If you interact with any other server on the network (meaning users on another server), all messages/attachments/metadata will be synced with all other servers by design.
Given the percentage of users on Matrix.org & servers they provide, some of your data will end up on Matrix HQ servers.
I had seen a post from the official element account that said something along the lines of "we send encrypted data to the government. If you don't like that, element isn't for you." Not word for word accurate but you get the gist. I didn't know if that statement only applied to the official matrix.org server or self hosted instances as well.