Like I said above, the Arofly does it, more or less, and calculates cadence from its speed estimates. But it has errors for example assuming coasting when one is pedalling or visa-versa.

The thread I linked to above had a few other bike computers that do it too. Yeah, buried in too many pages. Each with their problems.

Strava provides watts, but only with post analysis.

If riding on reasonably flat roads, then I believe one could use pure speed for pacing just as well as using power. Just learn what it feels like, and your personal endurance at 15 mph, 20 mph, 25 mph, or 30 mph. So, use speed and what the ride feels like while riding, then look at Strava later..