[sufficient]

I came to a similar conclusion: U2F hardware is the way to go. For some people, smartphones are becoming the only device they use. However, I am not fully convinced of using the device itself as a U2F key. Then it's no longer a two-factor solution. Thus, I envision the use of U2F hardware with mobile devices as the future of authentication.

Unfortunately, it is still difficult to find the NFC "sweetspot" at the back of your phone. At Cotech, we work on a Hardware Security SDK that solves this and works independent of Google Play Services. It brings support for U2F Hardware over NFC and USB to Android phones: https://hwsecurity.dev/fido/