I'm doing a desk-review project where we're required to analyze the findings from existing research. Most of my articles have used thematic analysis to analyze their data. Is it appropriate/rational to also analyze their themes using thematic analysis?
As far as I'm aware thematic analysis is designed to be easily applied to many different forms of research, including analyzing themes from prior research studies, so I would say that this would be viable. I am not as well-versed in thematic analysis as in other qualitative research approaches, though, so please take what I say with a grain of salt—if a better expert in thematic analysis disconfirms what I say here, then you should definitely trust their word over mine.
You can use thematic analysis on documents such as government publications not quite sure about using it on the reports of other studies unless your research question is specifically about how different researchers approach an issue.
Combining the results from a set of qualitative articles is known as Qualitative Systematic Review, and it is possible to use thematic analysis in this process. Here is a link to one such article, and you can find more in Google Scholar.
Article ‘Methods for the Thematic Synthesis of Qualitative Research ...