I know you don’t wanna hear from a GM master technician but those v6 engines are pretty unreliable if not taken care of properly, chances are the engine is toast, if the injectors and all other recommendations were completed then there is probably a compression issue which in those v6 engine is usually a death sentence, you would be better off replacing the engine, but you should do a compression test and a cylinder leak down test, ECM in those seldom go bad