[Peaker]

But you can do this without S-expressions.

DLang, for example, has free-form macros that they weirdly call "mixin" (as in, mixing in some text or declarations into the AST, syntax-wise).

Each mixin must be a valid AST subtree on its own, which gives the same guarantee that paren matching prior to macro expansion gives you.

Then, the compiler can interleave:

     parsing

  -> evaluating mixin strings
  -> resuming parsing of the mixin subtrees
  -> evaluating the deeper mixin strings
  -> ...

You get the power of Lisp macros, but without the Lisp syntax that is unattractive to many.