The use of terminology is sometime confusing. Both text analysis and semiotic analysis deal with the social system, which itself is a system of meaning relations (as Halliday holds) . In this regard, SFL, among other linguistic theories, offers a good means to do the analysis one is after. In other word, having an approach, combining both these questions simultaneously , is recommended.

Loretta Fernandez 's paper (2018), Qualitative Interview Analysis: The Use of Systemic Functional Linguistics to Reveal Functional Meanings, provides very insightful ideas concerning DA in general . It may be of help for your project.