I'd also look at RunnerUp - it's open source and runs on Android. Here's a link to the file implementing garmin export.
At least for some open source apps, GitHub provides for a decent changelog. Examples:
Signal: https://github.com/WhisperSystems/Signal-Android/commits/master
RunnerUp: https://github.com/jonasoreland/runnerup/commits/master
My app of choice on the rare occasion that I record with my phone now that I have a running watch is RunnerUp. The UI is super basic, but it's open-source (meaning completely free, ad free, and no tracking), and provides a LOT more options than other recording apps. The lack of bloat-y features also means it's the least power hungry app of this kind I've used.
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The audio cues can be set to repeat at any distance or time interval, with the HR audio cue options being:
It has all those options (total, interval, lap, and current) for pace, speed, and cadence too, and you can select as many of them as you'd like. For your purposes, I guess you could set an audio trigger and have it read off your current HR every 30s or something like that. I use it with my Wahoo Tickr, which is the HRM I'd recommend if you just want basic HR data as it's the cheapest HRM on the market from a reputable brand.
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Aside from the learning curve with the UI, the only issue is that it's only available on android. If you have an iPhone you could use the Wahoo app itself, but the minimum time trigger for audio alerts is 1min, and there are far fewer options to customise it.
I personally like RunnerUp. It's open source, which is cool, and it can upload your runs to Strava, MapMyRun, Nike+, Runkeeper, Garmin, and even directly to Facebook.
It's under active development, so it'll only get better.
Open Source und für Android: https://github.com/jonasoreland/runnerup
Kann man mit verschiedenen Services nutzen, die tcx/gpx unterstützen. Manche Seiten erlauben auch das direkte hochladen.
it's not on F-droid anymore, because the app has support for the closed source ANT+ sensors, but RunnerUp is pretty good. There are configurable audio cues, configurable training with intervals, support for various accounts. Te interface is a bit old but functional