This, lisp is perfect for representing arbitrary data, nesting is just another sexpr, easy to produce, easy to parse and easy to debug / reason about

When I did my degree, the years prior to mine had some flexibility choosing the implementation language for compilers class.

Lisp and Prolog were forbidden due to how easy the whole exercise would be.

I can appreciate this answer, but I don't think it's really what I'm asking.

I think I'm more looking for some kind of standardized struct definition that translates easily to llvm IR and is flexible enough for a wide variety of languages to target.

Something like this: https://gist.github.com/thomaswp/8c8ef19bd5203ce8b6cd4d6df5e... (Which doesn't meet my criteria because AFAICT isn't used by anything, but is reasonably close to what I want) or this: https://docs.rs/sap-ast/latest/src/ast/lib.rs.html#1-83 (which seems specific to SAP, I would like something more general)