I don't believe Apple will be updating iPad mini in September. It wouldn't make sense for them to provide any updates for the iPad mini alone, when they just had the conference back in June that was primarily about the iPads. The iPad mini could be likely updated within 2018 somewhere.
Not like an update takes much time - they spent 10 seconds in September 2015 announcing the iPad mini 4. If they were waiting to put an A10 (not X) in it then September would have made more sense than June to not steal light away from the pros. Spending 10 seconds saying the mini got an update while the new phones have A11 means no one but people that actually want a mini will even register it.
But yeah, March/April 2018 is also a possibility, that's when 2017 iPad got released too. Beyond that though, and I think it's done. Too many features in iOS 11 requiring A9/A10 and to not update two year+ tech (with three year old chip) by then to support them is a definite nail in the coffin.