Q: Would it be against research ethics if I write several research papers based on the data from one survey?

You seem to have described your scenario quite adequately. Based on the details you have provided, yes, this would be a case of salami slicing, and therefore, unethical to do.

Salami slicing basically involves publishing the findings from one study (involving the same research question, population, and methods) as multiple papers. This is typically done by researchers to meet their publication targets. Although your study involves looking at different diseases, that seems to be more of a categorization. It would be best to proceed with writing and submitting only one paper for now (unless you believe there are multiple research questions that you can indeed publish as multiple papers). If you later find that there are some additional perspectives you can share, you could do so by writing it as an addendum to the previous paper. If you are still not sure about how to proceed, you could check this with the editor of your target journal through a presubmission inquiry.

Now, you have mentioned the concept of ‘companion papers.’ That may be understandable if you were conducting a case series, that is, multiple case studies, which however doesn’t seem to be the situation here, as you said this is a survey.

For more insights around the issue of salami slicing, you may also refer to the following resources:

• The importance of avoiding salami slicing in publications: A case study
• Will using the same sample group in two publications be considered as salami publication?
• Is using similar methods in two different papers considered salami slicing?

Hope that helps. All the best for your paper!