I've long since known that define is scary and should be used with caution unless you know for sure how your implementation handles it. Out of interest, I recently opened up R7RS and read all that I could find about define and nothing gave me the impression that any of it is implementation dependent. Have I missed something or is define no longer implementation-dependent in R7RS?
You seem to be reading something into the answer you linked which isn’t there.
define has always been well defined, just as well-defined as let is. Most people choose to use define only at the top level of modules to create top-level bindings, but that’s a stylistic choice — it’s also capable of creating local bindings, like let is, if you use it inside and at the top of an ‘internal’ body, such as inside a procedure or a let or similar. Multiple defines in such a context are technically equivalent to letrec*, as another answer noted.