This is good logic. Keep my primary phone small and have a secondary for the Mavic. I may do this...I may even make the 6plus my "work" phone. Would give me an excuse to keep the darn thing charged, anyway.
So, how do you buy a used 6plus and know that the battery on the thing isn't already shot? They clearly have a fixed life...
If you want to buy a good iPhone on eBay my first recommendation is to stay far away from the big dealers that have hundreds or thousands of sales. Look for the small guy with 15-50 sales, 99% approval or better and is offering his own personal phone at a reasonable price. The best ones come from guys who have 3 or 4 old cases to throw in and they say the phone does not have a mark on it. That is typically a well cared for phone and the battery should give you another 2 years of service. My 6+ can handle 3 flights. It is the luck of the draw but your odds go up significantly if you don't deal with a big seller.