Q: For my follow-up study, do I need to write the synthesis again or is only the reference enough?

Hello again, Nadia. :-) We hope you would have gone through the response to your previous query. In fact, it seems this query is a follow-up to that one. If so, the purpose of this paper is clearer now: to publish the data for the characterization. That is great.

To answer your query, no, you don't need to write the synthesis again - you can simply provide a summary of it, along with citing/referencing it. For more inputs, you may refer to this similar query by another user (though from the field of case reports): Would a follow-up case report be regarded as duplicate submission or salami slicing? As suggested in this query too, it would be great to connect with the editor beforehand as this is a follow-up study. All the best again!