Also curious how this would look

It could look like this: (* (+ "a" (* "b"))) for: "(a+b)" [Perhaps you prefer not to re-use + and * for this.]

Sure, it's somewhat more verbose for this toy examples - but every intermediate expression (and the whole thing) is a Lisp-Object in its own right.

You would not even need macros.

Edit: There is a place for macros here to make things less verbose. Just write a macro that 'compiles': (* (+ a (* b))) into the form above.

For convenience you can offer a function that builds RegExps out of strings in the usual way.