I would advise that if you know upfront you will have to handle high load and do things like db-sharding, you should not be using Python and Django.
Python is slow and Django has a lot of limitations that makes it not suitable for this kind of scenario.
For the database, learn to write raw SQL. It will make working with the database more pleasant. And consider using a compiled language for the server application.
Think about what you need from an ORM? I think the most useful thing a database api can provide is to automatically convert a row from a database to a struct. Depending on what language you use, there will be a library that does this.