My site and properties from external sources

We have an on premise SharePoint 2013 farm.

In setting up mysite, we will only retrieve username and picture from AD, all other properies for profiles we need to get from another database that is not a part of SharePoint.

We have more than 2000 users.

I am wondering how to impelement this and about what kind of things\risks I need to consider.

How will the system understand that data from a database row will be mapped to the correct user?

I have given BCS a thought, but an external list will be to large.

Please advice.