I am just wondering if this is the best, lightest device to accomplish this.

Based on your description I would say no, not for the most common backpacking situations, where you are only hiking for relatively short periods, like a week, and then come home.

Even if you were hiking one of the long trails like the PCT or AT, in order to justify this particular setup you must imagine resupply situations where you were 1) able to buy AA batteries, but 2) unable to locate a wall outlet to recharge your rechargeable AA batteries that come with the unit. This would not be the usual situation when you resupplied.

In the most remote areas of the PCT you won't be getting any cell reception anyway, so you could preserve your battery power by leaving your phone off most of the time.

What is your intended use?