> I've also had them convert to markdown something like an excel formatted document.

This look like a task where the LLM would be best used in writing a deterministic script or program that then does the conversion.

Trusting a LLM to make the change without tools is like telling the smartest person you know to just recite the converted document out loud from memory. At some point they'll get distracted, wrong, or unwittingly inject their own biases and ideas into it whenever the source data is counter-intuitive to them.