So you spent a huge chunk of change on a phone and got a fraction back. Still seems... an expensive way to go about solving the problem.
Depending on your comfort level with critical-security-updates-only, the 6 is still useful. Though I would argue that being on a normal OS update stream is the right answer these days. Everything is broken, as a rough approximation.
The older phone batteries are pretty straightforward to replace, and make a big difference.
There, fixed that for you.
If you're looking at TCO numbers, I'm really not sure I can agree with you - mostly because modern Android devices have climbed in price to be comparable to iPhones, and it's fairly rare to get an iPhone that turns out to be a turd. Android devices are really hit and miss in terms of build quality, and a depressing number of them end up with fatal flaws that lead to a remarkably short service life.
Yes, I'm bitter about the Nexus 5X, home of the bootloop. But my iPhone 6S has substantially outlasted a pair of 5Xs, one of which has been back for warranty replacement once and is dead again, and the other is just a scrap device for talking to the car.