Hi @Matthew_Petrie,
the APM Server generally supports sending an error.exception.cause which should point to the original error. If this information was sent up, the grouping keys would differ based on different causes. Unfortunately the nodejs agent does not yet support sending causes.
You were mentioning that error messages differ. If any other information is given, then such messages are not considered in the grouping key logic, to avoid creating unique keys per IDs or names. Since in your case the errors provide a stacktrace the messages are ignored.
Looking at the nodejs agent's API I was wondering if it were an option for you to send the error message as parameterized message which would be considered in the grouping key logic.