I've got geometery data in a SQL Server database. My goal is to get it into GeoJSON format that my C#/ASP.NET server can offer to the client.
What are my options?
I apologise for my misunderstanding your question because of lack of information. But as I understood, your need is that your server can gets the SQL data into GeoJSON?
I would suggest GeoJSON.NET from github for your server to get the data in GeoJSON.