As suggested in one of the comments, you could use Word Automation. You are already using c#, so running and controlling Word instances is pretty easy. I have done so in the past using VB6 and Java as well. Works pretty stable, and you get high quality RTF with very minimal effort.
Other routes could include taking a long way like converting your docx to DITA or DocBook and use their toolkits to make HTML and PDF out of it. The PDF route is probably using XSL-FO. With the proper XSL-FO renderer, generating RTF out of it should be merely selecting output format RTF instead of PDF.
HTH!