I've done quite a bit of remote MTB/gravel riding/touring (e.g. much of the Great Divide Mountain Bike Route [1]), and have settled on loading GPX tracks onto a Garmin head unit. Turn-by-turn is pretty useless, because the notion of a "turn" is too ambiguous to be useful. Anyway, with a GPX track loaded the head unit will display the path, and you can tell when you deviate significantly. Oftentimes the turns are onto nearly invisible tracks; situational awareness will sometimes save you, but expect to miss some turns. This may sound unreliable, but it's a huge step up from everything else I've tried.

[1] https://www.adventurecycling.org/routes-and-maps/adventure-c...

I use gaiagps. Its very good for hiking and biking. Should also work for mtb. Had a garmin watch ( fenix 5s) once and that thing sucked hard compared to my phone. Its like a bad phone that can measure your puls.

I use Komoot. But it is not specialized on MB you can hike/walk/run with it as well.