That's a good choice of USB power, so it isn't the problem. And it's also OK to use the same cable for both. I have about a dozen Apple Lightning cables, some 5 years old, and only one of them has failed after getting pinched in a drawer. I use all of them with all 4 of the iOS devices in my family.
I can't really answer why your cables are failing; I've given a couple of possibilities, but it should be kept in mind that the MFI certification only says that the cable meets Apple's interface standard (which requires that it have the right microcircuits in it) and that it won't damage the phone. (See the link below about damaging the phone.) It does not certify the quality of the cable overall. And further, Apple does not test the cables; it provides a test suite to the cable manufacturer, and the manufacturer certifies that they ran the tests and that a small number of test cables passed.
https://motherboard.vice.com/en_us/article/bj34z5/why-counterfeit-lightning-cables-kill-iphones-cheap-iphone-chargers