Probably not what OP is looking for.

But I'm also happy with my Garmin Instinct. I use it disconnected from my phone, it does everything I need offline and stand-alone. To add tracks for navigation I just connect it to my linux laptop and drop the GPX file into the NewFiles (or whatever it's called) folder on the watch. I was surprised how well it works without official apps. The only thing I used the app for was to update the firmware when I bought the watch.

Also, fittrackee looks promising - thanks for the tip!