As suggested in one of the comments, you can use Word Automation. You are already using C #, so starting and managing Word instances is pretty simple. In the past, I have done this using VB6 and Java. It works pretty stably, and you get high-quality RTF with minimal effort.
Other routes can include a long journey, like converting your docx to DITA or DocBook and using their tools to extract HTML and PDF from it. PDF traffic probably uses XSL-FO. Using the proper XSL-FO renderer, generating RTF from it should just be the choice of RTF format instead of PDF.
NTN!
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