The Garmin won't be (much?) more accurate than your phone if it has a good GPS chip. You can take a close look at some of your tracks to see...

Bluetooth hits the battery on your phone pretty hard and that's how your phone will communicate with all those sensors. BLE is much better, get that over BT when possible if you go this route. Having the screen in is also a huge battery drain. So is GPS. But if you want to see your speed and power and HR you'll have your screen on to do it. People use the Wahoo head unit to have their phone screen turned off.

The Garmin will give you real time data, there's no difference from your phone there. Battery will last much longer, but if you're touring neither one will get you across the country without being charged. The Garmin battery will last through a weekend with a lot of riding though. And of course it does maps and nav when you don't have cell coverage.