It's up to Mueller to determine the end. If he had provable impeachable offenses what would he gain by continuing on?
Any other impeachable offenses or criminal indictments against Trump or anyone else that might come about as a result of looking into what happened from start to finish?
I don’t understand why you think Mueller’s finding of impeachable offenses should be a kill switch for the investigation. This case is of a much bigger scope than just Trump himself.
Several Trump campaign officials have already plead guilty to lesser charges under the condition that they’ll talk. And there will be more indictments to come.