If Trump doesn't shutdown gov't over immigration on Sept 30th then it's probably over as far as sweeping reform goes. There is no better time to do it. Make it the central issue of the mid-terms with an unfavorable map for the Dems. Take a vote, even if your own party votes against it then that is what it is, atleast people will know they need to vote those people out in the future. There is no good strategic reason to push this back until after the mid-terms if you truly wanted to change the system.