Your suggestion RE landmarks tallies with an issue I've been grappling with: how does a user actually run with the route that they've just planned? Yes, you can export the route to a GPX file, and some users will use that on watches/apps to guide them, but it's a bit clunky and isn't the kind of thing you'd necessarily do for a short 5km run in a new area.
I'd rather avoid building full run tracking functionality, as it'd just be repeating what so many other apps do (and often very well). Perhaps the answer is to enable easy syncing of the route to your other apps. Strangely, Strava (the most popular) don't allow importing of routes via their API.