This app was mentioned in 3 comments, with an average of 0.67 upvotes
I would give this app a try from the play store. I have zero experience with this app but it seems to do what you're requesting. Looks pretty robust and has a web front end too. https://play.google.com/store/apps/details?id=com.greenalp.RealtimeTracker
I use this app:
https://play.google.com/store/apps/details?id=com.greenalp.RealtimeTracker&hl=en
If I'm heading out riding somewhere I'm not super familiar with I use that app. It lets you type in some emails and generates a link that gets sent to those people. You have to be within cell service in order for it to work (obviously). The few friends and family members who I've used it with said it worked perfectly. They could see my little dot moving around the trail.
jeez, for that price you could buy enough portable phone chargers to track a multi-day tour. i also can't see there being much payoff in having your phone ask another device for its position instead of using its internal chip. even if bluetooth takes way less power than GPS i couldn't justify paying $400 for a redundant screen, chip and battery to power them.
besides that, if your phone battery does die, your garmin is offline.
i'd recommend trying the apps posted here first, to make sure you really need an external unit. i'm on android and i use Real-Time GPS Tracker.