The problem with using markdown for this is that it's unstructured, so when the LLM does calculations on it, extracting data is error prone, you're never sure what exactly gets extracted, and it's then a hassle to verify (like the author had to do).

In my app Superego (https://github.com/superegodev/superego, shameless plug) I use structured JSON documents precisely for this reason, because with a well-defined schema the LLM can write TypeScript functions that must compile. This doesn't guarantee correctness, of course, but it actually goes a long way.

But doing my taxes was a use case I hadn't considered, and it's actually pretty neat! I'll be trying it myself next month (though I'm not looking forward to it).