That was more like motion detection and was indeed useless. The S8 uses actual iris and facial recognition to unlock the phone and it actually works really fast. I would probably use it more than the fingerprint sensor.
The just released/updated Gigabyte Aero 14 with the GTX 1060 is really good, but it's 14" and probably outside your budget. Thin, non-gawdy design with good specs. Most importantly, it has a huge battery for very good battery life.
I would say play it safe and have an Apple store do it, but that's more expensive and maybe not worth it for an older phone like that. In that case, iFixit seems reputable for battery replacement kits.