How to handle an increasing number of increasingly divergent data structures?

I seem to be having to accomodate an increasing number of different schemas and data structures that are also hard to convert between going in certain directions. I don't have enough experience with professional software development to really know how to go about analyzing all of it, or what to look at doing.

I started off with designing a schema for generating the initial data in a way that it would come out of my api ready to be converted to xml compliant to a stringent DTD schema. But then I had to create some intensive functions to convert it to the schema needed for exporting to CSV, and the XLSX, and now I've got html table components too.

I thought about changing my base schema for my api, but that would require a redesign of some modules I spent weeks on, so I don't know if that would really be the best or most efficient course of action.

See my related question for examples of the schemas/data that I'm working with.