Not had chance to have a proper look at your app, but looks interesting.
As a headsup I’ve been using TrackMe since Windows Mobile was a thing, you should have a look at it for some inspiration. It’s not exactly the same, but there’s broad cross-over. Basic difference is it uploads a .kml file to a server, which is then accessible on all sorts:
http://www.luisespinosa.com/central_eng.php
I haven’t explored it in ages, this thread prompted me to have another look, so I don’t know how good it is at alerts etc, but if you set it up then add your .kml file to Google Earth it’s pretty cool.
One thing to consider is that if someone is waiting for a text to say you’ve passed a point etc, what do they do if you don’t pass the point. And it’s pouring down. And you’re on your own. And the point was the top of Nan Bield etc? With a live stream you can at least see when data issues have stopped an update. If it causes partners to worry unnecessarily then I imagine a lot of people wouldn’t use it, so cracking that would be a good idea.
Anyway, good luck with it, definitely interested to see how it develops.