C# Data Analytics Feature Integration

I have an existing C# solution that now needs a data-analytics layer. The goal is to pull information from three sources—relational databases, external APIs, and incoming CSV/Excel files—then process it into meaningful, queryable insights.

Here is what I need from you:

• Build or extend the data-access layer so it can read from SQL-based databases (Entity Framework or Dapper are both fine as long as it stays async and testable).
• Create lightweight connectors that call the specified REST APIs, handle authentication, and normalise the received JSON.
• Add a parser for CSV and XLSX uploads with clear error handling and a small validation routine.
• Combine these inputs into a single analytics service where I can run aggregate calculations, trend detection, and ad-hoc LINQ queries from the application code.

Acceptance criteria
• One unified interface (e.g., IDataAnalyticsService) that exposes the core methods.
• Unit tests for every data source plus at least two integration tests that prove the full pipeline.
• Clean, commented C# (.NET 6 preferred) and a short README outlining configuration steps.

If this fits your skill set, let me know how you would structure the solution and your rough timeline.