I have an existing postpaid contract that will end on 6 Sep and will incur a $35 early termination fee should I choose to cancel. Will I still be able to port?

Might be a bit too late to answer but it's not possible to purchase Giga earlier and terminate your existing plan later. Once you have purchase the Giga plan, your plan with another telco will be terminated. I guess you can calculate the savings you will save considering you terminate and pay $35 or pay the next best plan $25 for 45GB. Personally I have paid the termination fee to port over as I will save $ overall. But do not worry, now there are competition among all the telco so promotions come quite frequently.

Don't think it works that way. Better check with giga online chat service.

Alternatively, you can port to like gomo/circle first on 6/9 and wait for them to promo again. (As these services no contractual agreement)