Is there a way to save a worksheet as RTF, similar to how I can with HTML using the HtmlSaveOptions?
Aspose.Cells is an MS Excel or spreadsheet management library to read, write and render MS Excel file formats, see the document on supported file formats for your reference.
I think for your needs, you may try to use Aspose.Cells to render MS Excel spreadsheet(s) to PDF/DOCX and then use Aspose.Words to render to RTF format from PDF/DOCX.