Most Cost Effective Way To Switch To T-Mobile

So I’m currently on AT&T and am about to switch to T-Mobile’s 2 lines for $100 plan. I’m curious as to how to make the best use out of various promos and such. I currently own my own Google Pixel with no desire for any other phone at the moment. My wife is leasing an iPhone 6S Plus from AT&T with about $400 left on payments.

The employee at the T-Mobile store said if we switch and lease a phone through them, then they’ll pay off the rest of my Wife’s iPhone. The thing is, my wife would likely want to keep an iPhone but there’s no point in getting an iPhone 7 right now with the 8 launching so soon. Well I see the Moto Z2 Force is on a BOGO right now so I figured maybe I could just end up getting 2 of those, my wife could continue using her 6S+ and I’d just sell the 2 Motorola’s and buy the iPhone 8 outright when it comes out.

Am I missing something here about how this wouldn’t work? Maybe the Z2 BOGO doesn’t work if you’re getting a phone paid off? The upgrade fee for JoD kind of steers me away from just getting a throwaway phone and then jumping to the iPhone 8 in a month.