AFAIR you can call any application program from SQL as a stored procedure call, as long as the respective user's authority permits. This removes http requests and complicated remote start of applications from the equation. Not sure how parameters are passed in such circumstances, I'm not very fluent in the SQL world of IBM i. But maybe this is a start.

With a project I'm involved, ODBC was used as transport because that's what was established for accessing data in the i before .NET became involved in that project at all. "Outdated" (as you call ODBC) should not be a reason to make your life more miserable by trying to eliminate it. Because "almost" is not 100%. 😊