If you normally take your iPhone with you on all your rides, you really don't need a Garmin as it is really a duplication of services. There are plenty of phone apps out there that will do what you want without the extra cost of a Garmin.

RideWithGPS does an excellent job of giving you turn by turn instructions. You don't need cell service for TBT to work since the waypoints for your turns are inserted as part of the file when the route is created. You can create your own GPX route file if you have a printed map or que cards of the route by going on the RideWithGPS website after signing up. Once the route is created, you can download it into your RWGPS phone app and all the TBT info comes with it.

I have an iPhone SE and I did a cross state ride back in October using it as my cycling computer. Our second day ride started at 7:00 am and went until almost 5:00 pm. I kept the phone on the entire time, never charged it during the ride and ended up with over 60% battery life left. I have three Bluetooth sensors connected to my phone.

If you go into the phone's Settings and turn off Notifications to all the apps that you don't care to get notifications from and also turn off Location Services for all apps that you don't want tracking you, it will extend your battery life quite a bit because those apps are not using those services in the background, even if they are not running. When you get ready to ride, force killing all your running apps (double tap the Home button and swipe up all apps that are running, except the cycling app) and turning off WiFi will also extend your battery life.

iPhone SE

If you don't plan on using any sensors on this ride and already have another bike computer for speed and distance, the battery will last even longer. If you keep the phone's screen for the ride, you'll probably need to recharge it during the ride or get a Wahoo RFLKT to view the data from your phone app. The RFLKT has recently been discontinued but there are still plenty of them out there and it's compatible with RWGPS.

Most cycling apps are pretty simple to use and many of their features are almost self explanatory. Like a Garmin, each ride is tracked by starting the ride by tapping a Start (or similar) button and ended by tapping an End or Stop button at the end of the ride. The ride data is then saved and you reset the app/Garmin for the next ride. It's really quite simple. The ride is automatically uploaded to the RWGPS website so you can share it with others and/or uploaded to Strava, if you want to do that as well.

iPhones have excellent GPS receivers that are as accurate as most Garmin bike computers. Both the iPhone and the Garmin receive the same satellite signals and must parse the same GPS Sentences to extract the lat and long positions and data they need to display the ride stats. Where accuracy comes into play is the number of satellites that are locked in and the way the software/firmware parse the satellite signals and the way they handle floating point decimals. However, you won't see that much of a difference where it would matter for the type of tracking service you are looking for.

The iPhone uses cell service as a means to download map overlays on rides from one overlay area to another. For example, when you start a ride, a map will appear of the area you are starting in. The actual area may be larger than what is displayed on the screen. As you ride, you may go out of that area, so the phone app will use the cellular service to download the adjoining overlay. When downloading a preexisting route, loss of cell service isn't really necessary for TBT instructions since that works with incoming GPS data and the existing waypoints in the downloaded file. If the Garmin unit has on board mapping, like the 1000, overlays are uploaded to the Garmin by the Garmin Connect app on your phone. Garmins without on board mapping don't require this because all they export is a file with the GPS data and the route and maps are shown on the website or on the Garmin Connect app after it has downloaded the map overlays.

I hope this helps you out. Garmins are excellent devices but with the advancement in phone technology, phone battery life and the many features of cycling software, they really aren't what they once were, although Garmin die hards will certainly argue that point with me.